I have learned not all rulers are the same either. When I taught first grade we learned to measure with rulers sent by the curriculum publishers. Identical to the color but, the first and last inch were the least accurate. Usually they are a little bit longer than the other inches. Example 1-2 might be 1 1/8, but between 2-3 would be exactly correct. When I measure I begin with 3 inches and cut on 10, this exactly 7" piece. Does this make sense? I wish I could find a wide metal tape measure the retracts. The ones carpenters use is bowed.