I was noticing this exact issue when I was creating wallpapers for my Pixel 7 a couple days ago. Made a bunch of photos the exact screen resolution for the lock screen (1080x2400) or wider ones for the home screen (2160x2400) and found they always previewed perfectly but zoomed in slightly when set. It did this on the stock launcher, Nova, and when set through Simple Gallery.
My fix ended up being to pad out the image 10% to 1188x2640 (needs 120 pixels extra top and bottom, 54 left and right) to fix the lock/single screen ones and oddly enough add the same amount of padding on the wide home screen scrolling wallpapers (final resolution 2268x2640) and then they end up fitting perfectly pixel for pixel. I assumed the wider ones would need to be 2376 wide at first as well but when looking at the edge I could see the border I added.
Once you know the trick then creating images to work for wallpapers isn't that hard but it really shouldn't require this at all, I don't recall this being a problem on my Pixel 3 I just switched from.