Interesting Uber commented on whether or not they manipulate the fares paid to drivers, but did not comment on whether or not they actively try to screw drivers out of bonuses.
If you talk to Uber drivers, you will find that they have widely believed this to be true for many years.
It's easy to think the drivers can't do stats. But if you get "10 rides in 6 hours and final 2 rides in 3-4 hours" enough number of times, it's not hard to believe.
I could easily say that example tracks with a normal shift starting in a busy period and petering off as demand drops moving away from peak hour.
If every Uber driver got the same number of trips every hour, at all hours of the day, something would be seriously wrong.