It's not. If a variable's true value is in the set {1, 2}, then both the set {1} and the set {1, 2, 3} are approximations. But if your abstract interpretation i

It's not. If a variable's true value is in the set {1, 2}, then both the set {1} and the set {1, 2, 3} are approximations. But if your abstract interpretation is supposed to pick an approximation, it matters which one it picks. Picking {1} would be unsound. "Over-approximation" expresses the fact that it's an approximation that is sound.