I know this is an older post however I have been so busy over the Christmas period that I am only now catching up.
I never use the measurements on my board when cutting, always use my ruler. My friend who teaches quilting told me when I was first learning that I should always measure with the ruler and to always use the same ruler or set of rulers for measuring and cutting the same project as mixing rulers during a project could also see measurement being off.
Was told that the measurements on the mats are not as accurate as those on the rulers. Have 3 sets of rulers (all different brands) and upon checking the measurements in each of the brands, they are all different, which enforces why I should always use the same set to cut a given project (unless is is a design that doesn't have a set pattern to it) I imagine there is no difference with the mats.