Amma said: "The black line on the ruler needs to be on the edge of your fabric in the same place each time or each cut will be a few threads off."
That's what i found when i first started cutting fabric, i had a ruler with lines on it, and matching the line to the fabric was number 1 hard, and number 2, too innacurate, that's why i switched to matching the lines on the mat, and not moving the fabric, there is no distortion in the size of your cutting strips then