An inch is an inch, no matter where it's measured. I think that the "difference" between rulers is actually due to the difference in how we are able to perceive the markings, not on whether the markings are accurate. Rulers have thickness, and the lines etched into them should be accurate on the top, or on the bottom, but not on a slant from bottom to top. (Sorry, that's hard to explain.) Many times I think it is more accurate to use the mat, whose surface is two-dimensional, rather than the rulers, which are 3-D. But it's not always as convenient to use the mat, since it doesn't have as many lines and it means moving the fabric more.