variable output at pump?

[SE18]

I've got an OASES Aquamax 2200 gpm pump and something I noticed is that during the warmest part of the day, the output of the pump noticeably decreases relative to the cooler part of the day. I don't have a flowmeter or anything like that but it is noticeable.

Is this something anyone else experienced? I'm trying to understand why it does that. I don't need it corrected, I'm just curious.

Thanks

Dave