Our default item number for an entry is the number (or letters I guess) from the back of the box. It was only until Nintendo started doing disc based releases (GameCube, Wii/U, Switch) that the full/true item number was seen on the back... this number is better for our uses than the shorter numbers on the backs of SNES, Gameboy, N64, etc.
Here is the dilemma when it comes to handling the database from the admin side. If there is no number on the entry and you make an edit to add one, if it looks correct, then it will be approved. BUT if you are changing an existing box back number to another number that is on a box side (or on the flap) it will likely get rejected solely because it is often difficult for us to verify this information.
Our database currently suffers the same issue most other sites and lists suffer, people only use the cart item number, which is why you will still see so many item listings with that number, in error. But it is a non-crticial error and we will accept having the cart number as the item number in order to have something on the entry.
Now if we had access to a master list of item numbers that not only showed the number from the back of the box, the number on the side and/or in the flap, and the number on the cart, it would be a lot easier than digging through Ebay and hoping to find the same exact release (and for Europe it is a big pain as some boxed items never show up, and whole regions never return results) in order to verify the info and HOPE+++ that the ebay seller is one of those ones that takes pictures of every angels and side.