frozen chips always take longer

Often I cook frozen oven chips and frozen fish (in batter) in the oven at the same time. The cooking instructions for the chips usually say 20 minutes (or less); the fish usually more than this. They require the same temperature, usually 220C Why is it that (if I follow the instructions) the chips are always undercooked and the fish is always overcooked? It's the same with any manufacturer's product; the coking times always seem to be wrong. I have tried using different positions in the oven but always the same result.