I believe this was done after taking a look at GPA and post-hiring performance at Google. This was not mentioned in the OP, but I guess they realized that there

I believe this was done after taking a look at GPA and post-hiring performance at Google. This was not mentioned in the OP, but I guess they realized that there wasn't any or, maybe in certain positions (non-research/entry devs/etc), a negative correlation between the GPA and performance.