I just grabbed these bags to measure them. This is what I got:
L30" x H12" x D9.5"
L30" x H12" x W16"
We don't rely on the manufacturers stats and do all of our own. I think the discrepancies you are seeing is the "double seam" that this bag looks to have on the bottom. If this bag is stuffed completely, it could grow to a larger width, the second seam being the bottom of the front outside zip. I hope that makes sense!
Hope this helps, let me know if you have any other questions!