Set a photo exactly at your screen's resolution and it often comes back slightly larger than you chose, with a strip of sky or a shoulder missing at the edges. On an iPhone this is usually Perspective Zoom, the parallax effect that shifts the wallpaper a little as the phone tilts. To move the picture without exposing a blank border, the system first enlarges it, and the enlargement has to come from somewhere.
The cost is a thin frame around the image, often somewhere between a few percent and a tenth of each side. A portrait shot with the subject centred barely notices; a picture with a horizon tucked near the bottom or a face close to the top edge loses exactly the part that mattered. Some Android launchers do something related in the other direction, sliding the wallpaper sideways as you swipe between home pages, which asks for an image wider than the screen.
There are two calm ways to handle it. Turn the effect off in the wallpaper settings, and the picture lands pixel for pixel, or keep it on and choose images with breathing room, where nothing important sits near the four edges. Then the background can sway with your hand and still show you the whole of what you picked.
