It is all marketing, if you can put #7 in there then it looks like you have more pellets on target. The problem I see is that at 40+ yards how much energy do each one of those #7 pellets have? Will they even penetrate the skull of the turkey for a kill shot?
The shells are not illegal, they are just not legal for hunting turkey in Ontario as they contain shot other than #4,5,6. I personally like #6 shot, tons of pellets on target but I am also keeping my shots to 30 yards, I enjoy that range and the birds are really dead, not just a little dead.