I think they are trying to keep a logo/topic of the day or week theme for there logo sort of like a time magazine cover shows what is going on in the world or what is interesting to society that week or day...or their team just wanted something to do to keep busy/ to earn them big paychecks...lol
I think it's changing because of the Olympics. It seems a little strange how often it's changing though.
They do it based on something going on. Such as Holiday, or a important event in history like 911. It could be even something like the begining of a season or back to school time.
It changes often, every holiday it wants to find something to match. So like on December it probably has some Christmas logo!
I guess I don't ever notice what logo they're using, because I either use the Google tool bar on my home browser or the Internet Explorer search bar set to Google as my default whenever I want to use Google at work. It's been ages since I have actually seen the www.google.com home page.
Just for fun. Every once and a while they will change it for fun.