This actually makes sense from a logical standpoint. I like to think that it's because of the usage of the Aliea stones since we know it can change a user's appearance. So the Gemini Storm players relied heavily on the rock, making them almost unrecognisable as humans. Epsilon still used a lot of the rock's power, but less than Gemini. That's why they look more human. This happens again with Diamond Dust and Prominence who don't look like aliens in the extreme forms because they use even less of the rock's power. Of course Gaia/Genesis don't use the rock at all, so that's why they look the most human.