Are stay-at-home moms worth more?

Salary.com released its annual Mother's Day findings on what it thinks moms ought to be paid. (Figure out your own worth on the site's Mom Calculator.) From its release:

For 2008, Salary.com determined that the time mothers spend performing the 10 most popular "Mom job functions" would equate to an annual cash compensation of $116,805 for a Stay-at-Home Mom and $68,405 for a Working Mom, down from last years calculations of $138,094 and $85,938.

Whoa. Stay-at-home moms merit more pay than working moms? How'd that compute? The website says it used proprietary software and these parameters:

The job titles that best matched a mom's definition of her work in both countries are (in order of hours spent per week): housekeeper, day care center teacher, cook, laundry machine operator, computer operator, psychologist, facilities manager, van driver, chief executive officer and janitor.

And:

The primary driver of mom's six-figure salary, however, remains the amount of overtime worked. This year, mom's overtime averaged 54.4 hours per week. According to the Salary.com survey, Stay-at-Home Moms work a 94.4 hour "workweek" - over half her time spent on the job is overtime. The Working Moms reported an average 54.6 hour "mom work week" in addition to their paying jobs.

Okay. I have no problem with the job of mom being highly valued, if only by some b.s. survey designed to garner the website some press. But what sticks in my craw is the devaluing of working moms and dads. We too perform those other roles—housekeeper, cook, shrink—but on top of schlepping off to bring home the bacon. I just can't figure out how a stay-at-home parent pulls more overtime than a working one. Someone help me out with this math.