It is because you have two monitors with different scaling, re your reply below. I have the same problem with my 4K and 1440p monitors as well (200% and 150% scaling, respectively). This has been a rather big problem IMO, so much so that I have not really used the scaling that much and instead relied on increasing font size.
The reason why is that games run through Xwayland, and X only has global scaling factor setting and not a per monitor one. Therefore they have to do some weird stuff that I can not adequatly explain, but which is the reason for the applications thinking they are on a lower resolution than they really are.
I actually did a benchmark yesterday and it doesn’t really seem to matter performance wise (I thought they might both upscale and downscale, but that seems to not be the case from my testing). Itis also possible to tinker with gamescope if any games do not display properly.