I can't tell what the leaderboards show? As far as I can tell they only show completion percentage, which makes the leaderboards seem pointless to me... 
 
Any ideas?

MS requires games have leaderboards AFAIK.  I don't know how people get over 100% though...

Leaderboards in general seem to be pretty useless (except among friends) because there's always a group of people who manage to hack or cheat in the game to pull off stuff like 21853235 headshots in 0:00 seconds. Or beat the entire game in 0:00 seconds. Or get over 100% and it's not a Donkey Kong Country game.

" MS requires games have leaderboards AFAIK.  I don't know how people get over 100% though... "

As Fudgeman7 said, it would be great if they had separate leaderboards for different aspects of a run-through. Like time spent and/or number of deaths. They missed out a bit, I feel.

It's sad that the leaderboards have become so distorted.  MS is very slow to act against the hackers (currently 80+ above 111%).
Apparently PlayDead has stated they can't fix the leaderboard issue themselves, but shouldn't a dev be responsible for coding in a proper limit?  It's just bad programming to allow +128 and -128 values to exist in a scale from 0-100.  If your scale goes to 111 on purpose, and doesn't break when someone makes it say 125, I think that's a development issue.

Every XBLA game needs to have Leaderboards, Achievements & Help (or how to play or something) accessible from the menus.