Stockbyte / Getty Images

It’s a galling and often cited statistic: women make 77 (or 81, or 82) cents to a man’s dollar. President Obama campaigned on it last year, announcing in an ad that “women being paid 77 cents on the dollar for doing the same work as men isn’t just unfair — it hurts families.” Everyone from Lilly Ledbetter to Marlo Thomas has repeated it. And there it is on Page 6 of Sheryl Sandberg’s book, Lean In:

Progress also remains equally sluggish when it comes to compensation. In 1970, American women were paid $.59 for every dollar their male counterparts made. By 2010, women had protested, fought and worked their butts off to raise that compensation to $.77 for every dollar men made.

(MORE: TIME’s Cover Story, “Confidence Woman”)

Then Sandberg drops the topic of the pay gap altogether (although she later tackles raises and promotions). For someone writing a book on how women hold themselves back — “by lacking self-confidence, by not raising our hands, and by pulling back when we should be leaning forward” — this is a big missed opportunity. As it turns out, about two-thirds of that supposed pay gap can be attributed not to institutional discrimination but to choices that women make. Here’s why:

Let’s first dispense with the fallacy that the pay-gap ratios so often cited are for women and men doing the same job. They are not. If they were, then a female marketing account manager making $77,000, while her male colleague with the same title and work experience makes $100,000, would have a very good case to sue her employers under the Equal Pay Act of 1963, which protects men and women from sex discrimination in pay rates. The pay-gap ratios don’t even refer to men and women in the same occupation.

(MORE: Sandberg Exclusive Excerpt: ‘Why I Want Women to Lean In’)

Take 77 cents to the dollar: that figure is actually the annual median earnings of women to men for 2010, based on data collected by the U.S. Census Bureau. The other figure you often hear, 81 cents to the dollar, is the average median weekly earnings of women to men for 2012, based on data released by the U.S. Bureau of Labor Statistics. In both cases, the comparison is on an extremely broad level “that doesn’t account for differences in occupation, in experience level and a lot of other things that affect income,” says Tom Nardone, the associate commissioner for employment and unemployment statistics at the BLS, who makes a point of adding such a caveat in the second paragraph of a 91-page report on women’s earnings. “We at statistical agencies try to be very careful about defining our data, but we can’t control how other people use the information.”

The weekly earnings data are for wage and salary workers only and do not include self-employed workers. That means most of them work more than 35 hours a week, which minimizes the difference in the number of hours men and women work in general. (Yes, men work more.) However, some argue that the annual earnings number would be more accurate because it includes bonuses and other types of compensation not captured in the weekly earnings. What kind of jobs get bonuses? Well, investment banking for one, where, according to the Equal Employment Opportunity Commission, women make up only 35% of all employees and 15% of executives and senior-level executives.

(MORE: Forget About Mentors — Women Need Sponsors)

Which brings us to the bringing-it-on-ourselves part. Your occupation greatly dictates income, and women disproportionately enter low-paying fields such as teaching, nursing and social work. One could argue that those fields are low-paying because they’ve traditionally been occupied by women who were denied other career paths and were therefore devalued by society and in economic terms, but regardless, if we truly wanted to narrow the pay gap, women need to enter more lucrative fields.

To be able to do that, women must choose to study subjects that lead to more lucrative occupations — information technology or economics over art history, for example. But they are not. Amazingly, the percentage of undergraduate computing and information-science degrees earned by women has actually dropped from 37% in 1985 to 18% in 2009, according to the National Center for Education Statistics. No wonder the Labor Department also reports that from 2002 to 2012, the percentage of female programmers dropped from 25.6% to 20%.

(MORE: Dominique Browning: More Ways Women Sabotage Themselves)

If you control for things like college majors and occupations, the pay gap, or the discrepancy between men’s and women’s earnings that can be attributed to bias and discrimination, shrinks down to about one-third of its size. This is what the American Association of University Women determined when it surveyed male and female college graduates one year after graduation and found that, absent all explanatory variables, even including a graduate’s GPA and how selective their school was and how long they were unemployed after graduation, the women made 93% of what the men were making. In other words, 93 cents to the man’s dollar. Not 77 cents. Not 81 cents. Ninety-three cents.

Sandberg is absolutely right that women face internal as well as external barriers in reaching parity with men in the workplace. But one of those barriers may be misinterpreting statistics in such a way that we underestimate how much those external barriers are actually within our control to change.

MORE: TIME’s Complete Coverage on Sheryl Sandberg