If you dont use Stake much anymore, it is reasonable to see that your pre or post monthly bonus is getting lower than before. I've experienced it sometime ago. I used to get $25 from the post or pre monthly bonus but there were few times when I get less than $20. It means that our wagering activity will also affect this bonus so if we do wager regularly, we will get the same amount all the time but if we do wager less, the bonus will be less.
Ofc. The post monthly or monthly bonus is calculated only according to the wager in the period.
I just wanted to clarify that there is no minimum, so it is not correct that you always get the same bonus when you wagered... the variable is the amount wagered.

That makes total sense if you look at it from the casino's POV. It makes more sense that you "reward" the active players or make it as attractive as possible. From dead accounts that only claim the bonuses, they have nothing... I'm surprised that you still get atleast something.