For a start, that rank is comparing you to the other players, so you can play a game and go down in rank if you do poorly. There are two criteria - one column ranks you according to your average score per game (with the top player getting 500 points, and on down to the bottom - but I am not sure what happens down there if there are over 500 active players). The column headed Avg Rank shows how you compare to other players when your average scores are compared. While your average may go up after a good game and down after a bad game, your rank may not change much, if at all - it depends on how others near you have performed.
The other ranking is according to the total number of points you have accrued during the month, again with the top player getting 500 points. This is shown in the column headed Point Rank. Every time you play, your accumulated score goes up. However, it may not increase by as much as players near you in the rankings, so your point rank could actually go down!
The overall leader score is determined by dding those two Rank numbers together, and is shown in the Rank Points column. In theory, that could be 1000 - in practice, it is usually a number in the high 900s - right now it is 989, and the bottom score is 37.
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(Editor in Humanities, Religion, Literature and For Children)
That's all, folks!