ash isn't red there are major differences, anyways one theory is that.
Have you ever noticed Genesect and Kabutops stand the same way? Well, their Pokedex entries (Well, Kabuto's and Genesect's, but Kabuto could evolve during that time) state that they live in the same time period, and since Team Plasma couldn't have known about Kanto Pokemon, they colored Kabutops purple and added the cannon. Thus, Genesect was born.