I think what he is getting at is if your tube will hold 5 @ 3" it will hold more than 5 @ 2-3/4" shells.
As far as I know, it can hold 5 of whatever it is chambered for. But I have never been questioned.
I have a 3.5" A5 that will hold 5 @ 3.5" shells in the tube. 2-3/4" shells it will hold 7 in the tube + 1 = 8 which is what I use for 3 gun.
I always have 5 @ 3.5" shells in my range bag just in case but have never been questioned ... yet.
Adriel explains it better that I can here:
https://www.huntinggearguy.com/tips/...pacity-canada/