Don't think I am knowledeable enough, but let me give it a try.

I assume this is a product test that measures deviations from the target value in N number of different conditions. By summing N number of values and divide it by N, you get the observed average deviation from the target for the product type, which is supposed to give a good estimate of the (population) average deviation for all the products.

This Japanese sentence says that the sum corresponds to the average. Although a sum is not an average, it can be used as a substitute, i.e., equivalent, of average if the number of values (N) is constant.

The last formula of this page might clarify what this sentence means. In this formula, however, the sum is divided by the number of values (deviations from the target value) to give the average.
http://www.mv.com/ipusers/rm/loss.htm