I always have checked my different rulers & mats with a metal tape measure...guess I've been lucky...have only had one ruler be off 1/16" at the start..after the 1"...all was exact. I do use my mats to measure...but like I said, I check them first. I haven't really had any problems at all. I think it's the manufacturing process...and the larger the mat/ruler, the more likely for there to be a slight difference in that first inch measurement.