... the model comes with 14 tire treads! Is this just a giant error nobody caught?
No error. The "extra" 7 treads are to be used as tire tracks (or skid marks, if blackened with a Sharpie) for your diorama. The glue tab at the end is for accurate positioning on the diorama groundscape.
It is of course very important not to confuse the tracks from the treads: the treads (starting from the top) are the 2nd, 4th, 5th, 6th, 8th, 11th and 13th, while the tracks are 1st, 3rd, 7th, 8th, 12th and 14th. The 9th was added to correct the error of printing the 8th so it could be used either way, and is to be used as a the track made by tread #5.