Women in full-time, year-round jobs still make 77 cents for every dollar men make, said the U.S. Census Bureau this week — a zero percent change since 2002.

Full-time male workers brought in median annual earnings of $49,398 in 2012, compared with $37,791 for females. That's almost an $11,500 difference in yearly median wages.

So why has the gap remained unchanged (with the exception of one cent fluctuations in 2003 and 2007) for 11 years?

Discrimination plays a role, yes, but "analysts caution that the annual earnings comparison is a crude measure of a complex reality," says Brenda Cronin in The Wall Street Journal.

Here, three complicating factors:

1. Women are less likely to negotiate salaries and ask for raises
In her book, Lean In, Facebook COO Sheryl Sandberg says one reason is that women tend to demure when it comes time to talk about salary. A study of new grads with masters degrees from Carnegie Mellon found that while 57 percent of male grads negotiated their salaries, only 7 percent of females did the same, says Sandberg.

You'd think this would be an easy fix: Women should just ask for more money. But it's a little more complicated than that.

"[I]nstead of blaming the women for not negotiating more, we need to recognize that women often have a good cause to be reluctant to advocate for their own interests because doing so can easily backfire," she writes.

This is because, for women, there's often an inverse relationship between advocating for themselves and likability.

"People expect men to advocate on their own behalf, point out their contributions, and be recognized and rewarded for them," she says. "But since women are expected to be concerned with others, when they advocate for themselves or point to their won value, both men and women react unfavorably."

As a result, "even when a woman negotiates successfully for herself, she can pay a longer-term cost in goodwill and future advancement."

Sandberg suggests women negotiate and ask for raises anyway — but that they do it with care. For example, she suggests when asking for a raise, women offer a legitimate explanation, like suggesting that someone more senior encouraged the negotiation, or pointing to industry standards.

2. Women work fewer hours at salaried jobs than men
By looking at yearly salaries, the Census Bureau's numbers miss out on a key factor: That women sometimes spend fewer hours at their jobs than men.

This isn't to say women work less than men overall. A study last year from the Organization for Economic Cooperation and Development found that when paid and unpaid labor like housework is combined, women the world over actually work more hours. But men still spend more time on the clock.

Saying a woman makes "'77 cents on the dollar for doing the same work as men,' gives the impression that a man and a woman standing next to each other doing the same job for the same number of hours get paid different salaries," Slate's Hanna Rosin says.

"That's not at all the case. 'Full time' officially means 35 hours, but men work more hours than women. That's the first problem: We could be comparing men working 40 hours to women working 35," says Rosin.

A more fair measuring stick would be weekly wages, she says. When you look at it that way, the gap inches closer, to 81 cents on the dollar. When you restrict it to men and women both working 40 hours a week, you can kick that number up to 87 percent.

The number is also impacted by "career interruptions" — a.k.a. having and raising babies. On Freakonomics, labor economists Claudia Goldin and Lawrence Katz explain their research tracking MBA grads over 16 years. Viewing them as "apples to apples," Goldin and Katz saw a "modest wage gap" (favoring men) for new MBAs, "and these observationally equivalent men and women work approximately the same number of hours a week. But that changes:

Fast forward 10 to 15 years, and the earnings gap between our male and female MBApples is about 40% for those who were observationally equivalent at graduation. But almost all of that huge difference can be fully explained by the greater number of career interruptions and lower weekly hours experienced by the women (mind you, they still work a large number of hours). One of the reasons for the large gap in earnings between male and female MBAs is that the cost of career interruptions is very great in the corporate and financial sectors. [Freakonomics]

Because male and female workers are paid much more similarly before the interruptions start rolling in, Rosin suggests being careful about how you interpret the Census Bureau's data.

"If this midcareer gap is due to discrimination," she says, "it's much deeper than 'male boss looks at female hire and decides she is worth less, and then pats her male colleague on the back and slips him a bonus.' It's the deeper, more systemic discrimination of inadequate family-leave policies and childcare options, of women defaulting to being the caretakers. Or of women deciding that [they] are suited to be nurses and teachers but not doctors."

Which brings us to:

3. Women tend to favor lower paying jobs
It's well established that women are more likely to pick lower paying career paths, Georgetown University economist Anthony Carnevale told Lisa Chow on a recent episode of NPR's Planet Money:

Women are overrepresented among majors that don't pay very well (psychology, art, comparative literature), and underrepresented in lots of lucrative majors (most fields in engineering).

And even when they choose high-paying majors, women often don't choose high-paying jobs. For example, math is a pretty lucrative major, and more than 40 percent of math majors are women. But women who major in math are much more likely than men to go into lower-paying professions, like teaching. [NPR]

It's not entirely clear why this is, but it's been pretty much stuck that way for the last half-century, says Shan Li in The Los Angeles Times. "In 1960, the top five leading occupations for women were private household workers, secretaries, sales clerks, elementary school teachers and bookkeepers," she writes.

In 2010, the top five are "secretaries, nurses, elementary and high school teachers, cashiers and retail clerks," she writes. Not a huge shift.