If you get your tires rotated at pep boys for free with your oil change, they will just do the 4.

I think thats how it happens, you don't realize that you should incorporate the spare, or someone else is doing it and are being lazy about it, or just don't know that jeeps do a 5 tire rotation.

Slacker mechanics. anything for a shortcut.

Then the owner realizes it at 15k miles, and there you are.