Eh, I don't see why that even matters, although, there seems to be another rank called special jounin before that of sannin (if it's a rank, I'm not sure). But anyway, the ranks are there to make sure that the ninja have attained a certain minimum in overall attributes. This is so there are benchmarks to make sure that ninjas are advancing at a decent pace, and to guage what missions they can take part in. That's what I think. But, for exceptional ninjas, well, they don't seem to have come up with a ranking system to cover them, but it doesn't matter, because for all intents and purposes, the current one accounts for them just fine.