Can someone help me get my head around monsters with HD modifiers? As I understand it, the modifier adjusts the total number of HP after rolling HD. So a HD 3+3 monster would roll 3d6 (for OD&D) and add 3 for their final HP, with an average of 13.5. This is compared to an average of 10.5 for a flat 3HD creature. That makes sense. the +3 is essentially an extra HD of HP, but not extra fighting capacity. What I don't get are things like an ogre having 4+1 HD. What difference does the extra hit point make, really? Or manticores and basilisks, both with 6+1 HD. I can see the point of a goblin having 1-1, as a way to get a 1/2 HD creature, but these tiny modifiers on relatively high HD just baffle me. What am I missing?