Actually, it's not. The crater shader distorts the underlying terrain. In this case the 1st crater creates a bulge. The second one creates another bulge on the edge of the existing bulge. If you look closely you'll see that' the height is evenly distributed on the bigger one. If you look straight down on it you'll see they're both perfectly round
It may look weird, but it's perfectly logical
Keep it up, you can really do funky stuff that way.
edit: attached a render so other people can see what this is all about.