This isn't exactly abnormal. SO is a big site with alot of fresh content, I'm guessing google indexes many thousands of sites at that rate. What's surprising to me is that it surprises them.
Google tends to reduce its crawl frequency for content that changes less frequently. Perhaps in SO's case, it struggles to identify recently changed content without crawling the entire site.
A few thousand older posts are edited a day.
Throwing comments into the mix more than doubles the number of "new things", and of course every question, answer, comment, or edit is displayed on multiple pages (generally, on the owning user's page).
tl;dr - there's a lot more page churn than you might expect.