This could be explained by the 250-300 M you refer to not matching the same distribution due to

1. this seems to be google ad network specific, not google services per-see

2. the analysis seem to only include users which do in general generate ad revenue, e.g. all AD Block everywhere users are not included in the distribution

3. given the lower bound I assume ad views which have no clear attributable user, and/or users with a very low and irregular amount of views, are not included (e.g. some mostly "offline" people, people mostly using an ad-block but sometimes somewhere still seeing an add, also it's G-Ads, so anyone using only FB, TickTock etc. would not show up I think)