As PaperPrincess said, the yardstick and tape measure should both be accurate. I wonder if trying to measure fabric with a stiff yardstick is throwing it off? I find it easier to use measuring tape myself. For the heck of it you could measure the yard stick with the measuring tape and see if the lines all match up.
Depending on how the ends of the yardstick and tape are, sometimes it's helpful to start measuring from the 1" mark instead of the 0" mark. The end of the stick/tape might not be even. Just remember to subtract off that inch! My measuring tape has a little blank area before the 0 so I don't have to do that, but I've had others where I measured from the 1" because the 0" was right at the end (hope that makes sense).