Well, first, I couldn't really find the topic that was talking about it, and I thought it was better to just make a new one and tell you about it. Sledgehammer just confirmed (well, some time ago) on space marine forums that
So, that explains quite a bit."Killing Blow" With the Hammer/Maul Weapons:
Many users have been talking about the "Killing Blow" and how it is not always a one shot kill. Well it won't always be... Why you ask? Space Marine takes in consideration of glancing hits, so you might think your hit was perfect, but if you're not straight on with the enemy it will calculate as a glancing blow and do secondary damage.