Most likely answer is observer bias of some kind. Weapons have a damage range and skeleton hp could be in the middle of that range.

It's entirely possible to hit a regular skeleton with an iron mace (which they are weak against) and not have it die in one hit, because you rolled low for damage, or hit multiple things, (like a wall, or rock, or another enemy) reducing your damage.

Also, defense rating for enemies? Does that exist or is it speculation to support the bias?