The reason you'd have slightly different measurements is how the manufacturer prints the markings. Different weight inks will give you different measures. I have the same problem with my cutting mats- 3 different manufacturers, so I have to be very careful on how I cut, because the lines are all different weights.
You also need to be careful to always use the same 'side' of the line. For example, I use the Omnigrid rulers that have yellow & black markings. The yellow highlights the black, and is wider, so I have to make sure that I'm always lining up the next cut using the same edge of the yellow marking. If not, by the time I'm done I could be off by 1/8". Make sense?