Why do I have a game completion rate of 87% when I have completed all games?

"Game completion" might refer to anything that triggers a game behavior warning (failing to start, aborting, letting time run out)

If you lose on time (by that I mean: with much time remaining on your last move), then you WILL shortly see the message that says "Warning, RahumWindsong. Letting time run out will result in a temporary ban."

@StingerPuzzles So, I'm a new user (about a week I think) and looking at the Advanced Search page, what I see is that you somehow knew (or believed) that the "Game Completion %" statistic is directly (and solely) tied to the "Result/Clock Flag" attribute for the game(s). But....how do you know? (Also, if I committed some other thing that @Cedur216 suggests may be part of that %, then this search will not find those games).

Well, I'm not sure, when I play Bullet I often get flagged by nature, but I don't notice my gc% suffering from that

Two things:
1. I don't think lichess has to clarify game completion percentage. It is what it says. If you fail to complete a game "normally" then your percentage will drop. I don't see how the details of the calculation are so interesting, just play all the games you start. No problem.

2. This seems like something that should be discussed when it impacts your life. For instance you log on and find you can't play chess because your completion rate is 20%. It's an issue then and you should contact lichess. But until there's something wrong don't worry about it. Just play