Be sure to keep your blade pressed against the edge of the ruler and make note of the angle of your cutting hand. The cutter might be angled slightly like this / toward the ruler at one end of the strip and like this \ at the other; both angles will affect the strip width and smoothness.
1. Next time you cut a strip, before you move the ruler and with your hand still on it, gently move the cut strip out of the way.
2. Look carefully along the cut edge of the ruler....do you see any threads of uncut fabric where you may have wobbled the cutter blade at an angle away from the very edge of the ruler?
Because I am also guilty of over- or under-cutting the ruler, I usually try to angle this way / every time.
Jan in VA