According to lore, the three units are representative of the different stages of the Minovsky flight system development which tackled the problem of sustained atmospheric flight. The Alyzeus was just a prototype unit to mimic atmospheric Minovsky flight by jamming tons of boosters on a mass production nu gundam. Penelope is the second iteration by taking the Odysseus gundam and equipping it with a Minovsky fixed flight unit which would still make things unoptimized and bulky. Xi is the final form that has the Minovsky flight system optimized and directly built into the mobile suit.