Why earning rate drops after 2 h?

Hello all,

I am curious about one thing, every time I start mining I get several shares, 25+ per hour. My block rate is always high.

After a couple hours I get 15 shares /h and go from 2 ish raven to 1ish per hour sometimes I am at 0,7 rvn/h.

My hash rate sits around 38mh/s with 71 pl with a 3080 ti.

I’m new to this so any help understanding would be helpful.