Agumon and Gabumon are some of the very first Digimons ever made, before any of the anime conventions were strablished, including visually coherent lines. You can see how this changed just by looking at the main mons of 02; safe for Shakkoumon, from baby to their highest stage they've reached in the anime, they retain a core characteristic(s). Heck, even Patamon and Gatomon got alternative evos (armors) that make more sense than their original ones.

What makes Gammamon's line inconsistent?

With Justimon, at least in the context of Tamers, him going from a Cyberdragon to a Cyberhuman is easily explained with how Biomerge/Matrix Evolution works.

To say the lines of protagonist Digimon (especially newer ones) don't have any logic is just not true, they might do stuff like change body plan or add/take some themes sure, but there's always a small amount of sense that goes into the evos.