My shotgun is designed for 3" shells. It will accept shorter shells, but I expect it's stated magazine capacity to be for the number of rounds of the longest shell it was designed for.
I've seen short "tactical" 12 gauge rounds designed so that you can get more of them into a tube mag. If they count that as a "round" and you can only have 5, then you probably can't have more than 3 3" rounds.
Actually, my point is that most manufacturers list their 12ga magazines’ capacities based on 2 3/4” shells. That’s the standard length. 3” and 3.5” are Magnum shells. Anything less than 2 3/4” is considered 12ga Short these days.
I also suggest you re-read your owner’s manual for whatever arm you have.