Cutting mats aren't always accurate, especially when they get older and more cut up/used. For larger cuts, maybe try the 2-ruler method of cutting.
My friend the quilt shop owner (you can imagine how fast she'll go through a cutting mat!) had a 1/2 yard sheet of plexiglass cut. They etched lines for an eighth of a yard, quarter of a yard, third of a yard, etc. Doesn't matter where she is on the mat.