My Fiskar and Omni and Olfa mats are pretty accurate, but not perfect. I have measured them with my Creative Grid and Omni rulers (yes Dunster, good point about 3-d vs 2D !!!) and my carpentry rules and I have done story-boards to double check. Story boards are when you take a flat thing like paper or thin wood stock and use careful fine-line-marker to mark dimensions. It is more accurate than rulers especially for big things. Same thing as running dimensions for the ruler on a map.
I use the mat lines and marks for rough dimensioning and lining up and double-checking to make sure my fabric hasn't mysteriously slid on the mat.
Part of this is if you stick with one way of measuring everything, you will be more consistent.