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The bulk of grimoire comes from getting, lets say 500+ kills of each type of enemy or getting 5000 kills with a certain type of weapon. The more grimoire you have, one could assume that you're experienced because you've done a lot of killing. I think.
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Makes sense
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It mostly shows the effort you put in the game. You made the effort to win a gametype X amount of times, kill an enemy X amount of times and collected every collectable in the game. It shows experience as well, to some extent. Not enough to rely on though.
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Cause... No reason at all. But under 3000 first account mostly means unexperienced in most cases.