Yeah, when I restarted my world to play H&H in order, the first two swamps I found had 0 crypts whatsoever, then the third one had 7.

If I had to guess, I'd say there's a narrowly higher chance that more distant swamps will have more crypts, since it's less likely for swamps to spawn near to the center of Valheim so they'll tend to be smaller and crowd out the crypt spawning algorithm.