CS10 is a class designed mostly for non majors, who are interested in Computer Science; CS61a is the first required class for the major. I know of a bunch of CS guys with their girlfriends in CS10. A lot of people who are already set on taking CS as their major go directly into CS61a, skipping CS10, and this accounts for a lot of the gender ratio difference in CS10. If you look at a class like CS162, you can see the gender ratio of the people who are actually sticking through the major.
This doesn't mean as much as graduation rates, etc.