The grit only works where it supports the tool. So as long as you rotate frequently it will just hit new 'high' spots while not affecting the low spots at all. By hitting the interior more often than the edge that's where you'll go deepest, it's more statistics than actual aiming for a some high spot or difference. The tool-on-top or mirror-on-top swap can help to adjust if you've gone too deep by favoring supporting the tool at the edge or supporting the tool in the center causing greater amounts of material being taken away at the edge.
Lots of simple processes can cause nearly perfect geometrical shapes to appear: bounce steel blocks on a vibrating drum for a while (and against each other) and you end up with steel balls that rival ball-bearings in roundness (but not in precision, they will be all kinds of sizes).
Yes, it's real. I don't have a video and I don't have a metal workshop any more but you can easily try this yourself if you feel like it. For added speed you could add some sand, and if you're just interested in the effect mix some rocks with some wooden blocks, that will speed things up quite a bit.
It's similar to how river beds tend to round the stones that get moved around by the water. Those stones start out as sharp bits of rock.
Lots of simple processes can cause nearly perfect geometrical shapes to appear: bounce steel blocks on a vibrating drum for a while (and against each other) and you end up with steel balls that rival ball-bearings in roundness (but not in precision, they will be all kinds of sizes).