I believe it's NFL uniform rules. If two players have the same last name, they must wear their first initial to differentiate them. If the players have the first letters the same, they may wear their first two letters. If the players have the first two letters the same, they must wear their full names, first and last, on the back of their jerseys. Hence why Tank Williams and Mike Anderson, amongst others, had their full names on the back of their jerseys recently.
It may even be full names if the first letters are the same, but I believe you are allowed to wear your first two letters.