We'd need more data, but obviously the attrition rate could remain constant for the whole while changing for some of its parts. Perhaps Google has gotten better at retaining non-entrepreneurial engineering talent, while getting worse at retaining entrepreneurial engineering talent.
But you aren't really basing any of this on any facts or even statements by involved parties, are you? This is just theorizing that maybe their rate is worse in some spots. Maybe their rate is even better among this last category. What is the point of idle speculation like this?
I have often wondered why some companies (even mature ones) actively recruit entrepreneurially-minded folks. It just seems to me that they would create trouble for you once they realize you aren't acting like a market internally, and are only willing to pay labor-market rates (except in discreet circumstances).