Think about it. Your tires are measured by the inside diameter. 700, or 27in, or 26in refers to the inside diameter of the tire (even that is not exact). Your cycle computer reading is based on the outside circumference.

I now own two tires, both 27in X 1 1/8 inch but different brands. The outside diameter of one is ½ inch larger than the other one. It’s so much bigger that I can’t use it with my fenders. ½ inch diameter means about 1½ iinches more in circumference (if I remember my geometry right). That has GOT to change you millage readings.

That's why the figures on the cycle computer instructions are only approximate.