I've heard that as rulers age, you should hold them up against a flat surface to be sure there aren't any bows in the cutting edge.
How else it would be off is a mystery to me, but I have heard that some people measure on the cutting mat, which can be a guide, but is not as accurate as a ruler.
Lastly, brands might be a hair differnt, so that might make a difference - try to stick with one brand.
Honestly, I think it's likely user error. I have found that my cuts are more accurate now that I use Creative Grids products. The lines are finer and those colored lines on other brands are not only too wide for me, they are distracting.