Why does my "Expected per day" changes so much?

My rig is running pretty smooth over the last 24 hours, and everything seems fine (hashrate was not under 740 at any point)

but the “Expected per day” moves between 0.230 to 0.340 , and this is a big movement. (the pool Difficulty is set at 1.25 on all miners)

since this is ETH (not USD) it should not move because of ETH price. so why does it move? and why so drastically ?

looks like i have 50-60 $ changes, but my hashrate stays the same

can anyone explain ?