I think they're looking at it from a an individual product perspective. % users of an entire company can be extremely different than % growth of a new product, within that company.
My naive assumption is that they have some neat simulated user/load system to help identify any scaling issues that might arise when launching a service, and the extreme % growth. That 3.8 billion was spread over 15 years.
A few million new people is less than 0.1% of the total. It's entirely within expected variance.