Why would they manipulate the numbers that way if it’s self-reported anyways? If you want to fake the numbers, couldn’t you just change the number and be done?

The people fudging the metric (developers) may not be the same people that are relying on the metrics (managers). Perverse incentives at play could easily motivate this kind of shenanigan.

It's probably quite difficult to manipulate per-user metrics after the data is collected.