A blank is not a zero

A blank cell means no value was recorded. A zero means a value was recorded and it was none. They are different claims about the world, and Excel treats them differently: AVERAGE skips blanks entirely and includes zeros in the division.

So a survey where two people did not answer gives one average, and the same survey with those two marked 0 gives a lower one. Neither function is wrong. The question is which claim is true, and only you know that.

The rule of thumb: if the measurement did not happen, leave it blank. If it happened and the answer was nothing, write 0. Filling blanks with zeros to tidy a column is how a dataset quietly acquires facts nobody observed.