Still it feels as if some mobs are tagged for spawning some type of loot. I know of a mob in wetlands that will drop blues eventually (within the hour) happened to me many times (made a nice buck of it, twink stuff). Have simillar things with a mob in zf, dropped an epic at least 3 times for me now. Another mob in shadowvalley dropped a recipe for me on 3 occassions now (bop though , and i was partied all 3 times, last time it was for me, cheer :P))

Think its a bit like the npc vendors and their not always available recipes, there is a time for it to drop/spawn and as long as you are in the time frame of it dropping and there wasnt somebody else to get it, its yours.

Bottom line from what i wanted to say, in my experience and humble opinion there are mobs with higher /blue/purple drop rate.