Because your piston stretches higher at high rpm (Centrifugal force) it reaches the top of the cilinder wall. It won't reach that at idle.
The cilinderwall has a crossed pattern from the factory, to allow the piston and wall to smooth into each other. This leaves a little excess metal, and if you don't get the piston at the top of the cilinder while this is scraping off, it'll form a bump a lil under it. And when you do rev it up, your piston will encounter a lil bump.
Most of the time it isn't really harmful, but it could seize because of it.