I'm at 98%, missing the speedrun achievement, and I see percentages from 102% to 112% on the leaderboards. What is the highest non-glitched completion percentage possible, and what adds to the completion percentage over 100%? Easter eggs?
The maximum legitimate percentage has been confirmed by Playdead to be 111. Percentages with 112 and more involves downloading someone else's game save, and no matter the reason, is against Microsoft's TOS.
Supercheats.com explains the 111 percent as follows:
You may also want to check this forum thread for some details about how they get 112 or more.