Good luck!

Tire manufacturers are notorious for bad sizing. Racers want the lightest gear, so tire manufacturers started claiming wider values than actual so they could say they have the lightest tire in its size. It's followed suit with quite a few non-race tires, but for non-racing rubber the problem seems to be batch-to-batch variability due to sloppy manufacturing tolerances. In an article from Velo News over the summer (about UCI changing the 34mm width max in CX racing) it was brought to light that one manufacturer has a tolerance of +/- 2mm. That means one batch of 28mm tires might be 26mm, while the next is 30mm, and they're within spec.