I don't think Google's hiring is increasing fast enough to explain the low rate. It looks like they increased about 10% from 2019 to 2020.
However, it's also unclear exactly how 1.1 number is calculated. It looks like it comes from PayScale, which doesn't give, as far as I can tell, a methodology:
Is that the median amount of time that all current employees have had with the company so far?
Or is it the median of all of the employees that used to work there & moved on, which would exclude people that have continued to work there?
Or is it some type of survival analysis that takes such factors into account?