They're doing an A/B test using Google Website Optimizer. You can see the Javascript footprint if you know what to look for.

Incidentally, this is the first time I've ever seen Google using GWO. It is not good software -- for one thing, this thread exists where if they were doing good A/B testing it wouldn't. I used to joke that they'd swallow broken glass before using it on a site which actually mattered to their business interests. Guess I was wrong. (Although they won't be putting it on any app anytime soon.)