The savings will not be cumulative because of interaction between measures. For instance, replacement AC unit might save 10% of the energy. Replacement windows might save 10% also. But when you do both, the new windows have now reduced the load on the AC, causing it to run less, so the savings from replacing it will be lower. In an extreme example, if all load on the AC was eliminated due to an efficient envelope, replacing the AC would save nothing since it never would be running.
← When I run the alternatives as individual measures, why do the combined savings for all measures not add up to the sum of each individual measure?