Why battlecruiser classification depends on its contemporaries

Battlecruisers can have armor as thick as an earlier generation of battleships while protecting a smaller proportion of the hull. Comparing them only with older ships can therefore obscure the intended distinction. G3 would look more like a fast battleship if built alone, whereas a contemporary N3 would make G3's battlecruiser role easier to recognize. A similar argument could have surrounded Iowa had the larger American battleships planned alongside it also been completed. Speed and belt thickness need to be considered within those contemporary design choices.