You did something then before disconnecting.
If you join and do nothing then you get removed totally so efficiency etc is worked out correctly when merging duplicates together (ie. player rejoins).
Nothing is defined by:
0 kills, 0 deaths, 0 teamkills
So if you joined, got fragged, left the server and rejoined then it would keep that previous player in and that player would have 0% efficiency.
You rejoin, get 72% ... well come merging time its Average(72+0) ... 36% efficiency ... sorry
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