The shells had radar proximity fuse, so they popped if you came close enough to a target.
They could puff regardless after a certain flight time, but that would make aimed MUCH harder. Remember, we are seeing a 2D screen representation of a 3D world, so its hard to percieve depth. By having the puffs occur only if you're close to hitting, it gives you important feedback about aiming - and that woudl disappear if the timed fuse went off regardless.