I'll explain why mirror matches are 5:5 through LOGIC.
Your argument is saying that because one Litchi can do massive damage to another that the matchup should be 6:4 or whatever. Here's the thing: matchups are under the assumption that both players have the same amount of skill. This means that while one Litchi can do a 85% combo, the other would be just as capable. Same with Arakune; if one Arakune's able to curse another, the other Arakune has the abilities to do the same, as well as the same capabilities if under the assumption that they both are at equal skill.
In layman's terms, in mirror matches both characters have the same moves, therefore the same tools, therefore the same gimmicks, therefore the same strategies, therefore the same type of gameplay, and therefore the same matchup.
[/edgeworth]