Most likely they have older tried and true TNF shells made prior to 2001. In 2001, YKK changed all their zippers due to issued with teeth damaging too easily. What this means is that a pre-2001 YKK zipper will only mate with a post-2001 YKK zipper on the left side. The right will not zip, which means if you have an older shell, you have to hunt down a 2001 or earlier Denali/Pumori/Salathe to zip into the interior.

I had the misfortune to work at REI in clothing in Fall 2001 and had many unhappy customers.