I never consciously noticed that, but you're right. From what I remember the proportion of women in my CS classes wasn't quite that low, but it was still south of 10%. 33% also sounds about right for non-engineering STEM majors in my (publicly funded, moderately selective) university in the early-to-mid-Noughties, though that's skewed upward a bit by a student body that's 60% female.

It seems implausible, though, that a poor professional culture regarding gender would skew numbers that heavily in a freshman CS class -- most of these students are going to have had no substantial exposure to professional IT or related fields beforehand. I think we're looking at something with deeper roots. Specifically, CS is linked to geek subculture in a way that the rest of STEM isn't: you might naturally consider a math major if you were undecided and your best high-school grades were in mathematics, but there's no such path to IT. You generally only go into it if you already identify with the culture surrounding it and want to be part of it professionally.

With this in mind it seems likely to me that professional IT's attitudes are largely determined by the subculture's, not the other way around, and that gender ratios in CS aren't going to change much unless and until the culture changes.