Some users are not receiving pushes randomly

After some internal tests with a lot of eligible users, we’ve noted that users are not receiving pushes randomly, even with all conditions met as listed here: Commonly reported WordPress Integration Issues & Fixes. I’m currently on free plan, but my quota usage is at 51% only, approximately 31k active users. What could be?

There are no system issues at this time. We track our delivery stats in detail every hour at an aggregate level (across 7k+ sites) and metrics are trending normal.