I've never tested it personally, but I wouldn't be surprised if the AI's for specific Civ's would sometimes conflict with a preset VC. For instance, I've never seen Alexander go for a Culture victory. In my games he always blankets the map with cities, goes big fast, and builds up a strong economy to buy out all the CS. Fast expansion is directly opposed to efficient cultural play. I'd imagine this also applies to the Mongols, Huns, and Russians too.
Beyond that, your guess is as good as mine.