That has to be it. It would have to have been something with expanded traits. Occult status is determined by a hidden trait that gives them all the abilities of the occult type, and there is only one slot for that. Without such alteration, the game cannot assign one sim two occult statuses. It's just not possible with how the game is originally coded. That's why when a vampire sleeps in a Cursed Sarcophagus of the Kings it doesn't turn into a mummy. Instead, one random trait is changed to "Evil."
One of my obviously vampire children just rolled a wish to make the house spotlessly sparkling (or whatever it is that Genies can do). He doesn't have any genie abilities, but most occult abilities don't show up until teen or YA.
I know it isn't possible to turn one type of occult into another (as in turning an IF into a vampire, or a vampire into a mummy), but I'm relatively certain that crossbreeding allows more than one occult status. I'm currently running a mod-free game, so it wouldn't be anything to do with that this time.
Edit: Okay, perhaps I spoke too soon. This sim seems to just be odd. He's still a vampire and can take plasma packs from the fridge, but his motive bars are all still green. He doesn't have any genie abilities, even though he rolled that wish to use one. Perhaps because I used testingcheats to age him up to YA? Exiting the game didn't change anything, nor did moving him to another house via edit town. He's got me stumped.