Originally Posted by ShirlinAZ

Even 2 rulers made by the same company can be different, as well as 1 ruler when it's new and the same ruler when it's 5 years old. There can be slight slippage during manufacture that gets past quality control. You may "shave" the edge of your ruler each time you cut, thus changing it's measure with age. There is also a difference if you cut through several layers of fabric or 1 layer. Ever wonder why the same size clothes from the same brand fit differently? It's the fact that several layers of fabric compress. The top will normally be very slightly larger, even more so as your blade dulls. Learned this in design school 50 years ago. Oh, I still use multiple rulers, and my cutting mat!