My rear fenders are 12" wide.
As for the discrepancy, wheel widths are actually measured (I think) from wheel bead-to-wheel bead (the inner side of each wheel where the tire actually mates to the wheel. Therefore, a 10" wheel (with lip or bead roll, for lack of a better term) may be closer to 11" wide overall. My 10" wheel, for instance, with its 4.5" BS and -25mm offset would then make sense (lip or bead roll equal to approximately .5" So, .5" BS + .5" lip = 1" or 25.4mm, hense the -25mm offset.