Does anyone know why pending BATs appear and disappear?

I have noticed these days that I usually see 30,655 pending Bats, and after a while they disappear from there, normally 2,115 pending Bats appear.
(Which in part I find the current payments very poor, since I hardly receive advertisements and those that appear almost do not give anything …)

I would not say no to that payment that appears without explanation, I just want to know if anyone knows anything about it on the subject.
Is it a bug on the platform? Will it be a pending payment? An award or what?
Since sometimes I feel that it is as if the browser rubbed that payment in my face to snatch it from me :v

Any moderator who can answer me please?