In some cases none, in other cases there are some. If not, they would lose the right to make the tape. So all the tape is 'pretty good'. Tape Manufacturers can make the tape much better than the specs, if they choose to. In other cases, consistency is the name of the game.
All manufacturers occasionally have a bad batch.
Consistently good product is what to look for. In some cases, there is no difference. It was a well known fact, when Panasonic first came out with M2 tape, that the tape was made by Fuji. There are many such OEM arrangements which are not allowed to be disclosed, in which one manufacturer makes tapes for another manufacturer. Reason is, each manufacturer has so only so many resources and capacity, and it is often less expensive for them to source the tape from a competitor, and put their name on it, than to manufacturer it themselves.