According to
https://i.redd.it/u1hd57batpky.png there is almost a 1000 PI difference between the highest base PI champ, Thanos (4801) and the lowest, Wolverine (3810). That's a pretty big spread, and it doesn't seem like the values are simply based on the power/utility/stats of a champ, otherwise Phoenix wouldn't be #4 on the list. Instead they seem arbitrarily assigned.
If all champs are supposed to be relatively fair and balanced between themselves (HAH! Try saying that with a straight face...), what purpose does such a gross disparity serve?