Respawn groups doesn’t mean they’ll respawn at the same time, just that some are usually found in different areas of the map. I’m almost positive you got KS’ed a few times there, I usually farm 1 channel until I get the first item I want, then change channel.
More often than not you’ll be able to find the same item in the other channels at a lower killcount due to passer-byers killing a few before you.
You could also get 2 from a single channel below the killcount number if there are multiple respawn groups.
These 2 points make DPK “not that bad, really”, but we don’t have all the info on mobs over lvl200lmso it’s hard to know which zones have multiple mobs or which are lumped together.