I've doublechecked and they work as described.
How to reproduce:
Go to the lower crypts, under the burned house. Find a large group of undead (Any group) and cast Undeath to Death on them. In the combat log, you'll see that only one or two (Depending on the spawns) have to make a saving throw. The rest of the group are completely unaffected and never make a save.
Go to the wasps in the Signal Tower. Cast Circle of Death on a group of wasps. Only one or two wasps have to make a saving throw. The rest of the group is unaffected and never have to make a saving throw.
While it's safe to assume that some wasps and skeletons in those areas are simply more than 8 hit dice, it looks like it doesn't always trigger against every spawn of the same type.
If there are three of the same type of skeleton, it often only hits one or two of the group (Unless it's a very weak spawn).