It's limited or Medium would run out of money due to the ones that are really active with engagement.

Every account only has a certain amount it can give out and it will slowly go down the more someone is active.

My earnings are roughly the same, but some days randomly go down or up without much reason.

I just assume part of it is due to my readers and Medium also controls the amount of earnings given out each day. So, some days might be better or worse than others.