However, modifying any calculator

Calculators are not modified, they are just interpreted differently, so that when trying to answer the question of what happens in a certain situation (containing certain calculators etc.) we get different answers depending on what the assumptions are. The situation is the same, but the (simplifying) assumptions about it are different, and so simplified inferences about it are different as well. In some cases simplification is unavoidable, so that dependence of conclusions on assumptions becomes an essential feature.