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?

Some of this is related to the way the internal tool reports your seniority. It tells you that that X% of all current employees were hired more recently than you. So during this particular time frame, when Google was really booming in total head count, that number doesn't really reflect the average time-to-find-other-opportunity very well. Its reporting on the wrong side of the tenure timeline to give you that information. Even worse, depending on how you use the tool, it can include contractors as well.

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.

The retention rate was widely discussed in the groups I was in. It was especially low in Retail Customer Experience (RCX).

In my teams, representing less than 20 people in total, firings were far more common that people leaving. Two business people (marketing focused) left, two developers transferred to other areas (early AWS), while four developers and one business person were fired over the four years (fulfilling the "making hard choices" performance review requirement).