### How does energy utilizability work in Open Studio?

So i have modeled a building and I am applying energy solutions to observe the evergy savings of each solution.

For the first energy saving solution i add insulation to the external walls of the building (Solution A).
For the second one i reduce the infiltration of the building (Solution B).
For the third and last one, i combine the two previous solutions, so i end up with a building with insulated walls and reduced infiltration (Solution AB).

When checking the savings of each solution i realized that the energy savings of solution A + the energy savings of solution B are less than the energy savings of solution AB. Does anybody know why that is?