Earlier quoted context omitted.
They're up to 1.1 years now? Wow! They were at six months for developers for most of the time I was working there (2006-2010). At three years, I was in the top 25% of seniority ("old farts"). By comparison, Overstock.com averaged five years for developers between 2010-2013.
I find that incredibly hard to believe, given Google's reputation. Also, if the average was six months, that suggests that a significant number of exits were happening even sooner than six months. Did you personally observe that to be occurring frequently?
There is a separate tool that tells you the seniority of the people that are leaving. And while there is definitely a statistical bump right after the 1 year vesting cliff, even a cursory glance shows that the average and median final tenure is quite a bit longer than 1 year.