What kind of math is this?

Osborne

If i am not wrong, highest pi should be your top champions right?

SirGamesBond

When you view your top champs, it shows highest PI (power index) champ first. Those are increased due to your masteries.

When someone else view your profile, just like in your screenshot (which you try to cover by cropping and is obvious), it shows your highest prestige champions first.

There's an option to check that in game in

Champions →sort → base hero rating.