I just looked at the source code.
So it takes your position ranking across the whole list of users based on points, calculates which percentile of ranking players (defined as players with > 100

) you're in, then allocates the rank based on which percentile you're in.
top 5th percentile = Diplomat
top 10th percentile = Mastermind
top 20th percentile = Pro
top 50th percentile = Experienced
top 90th percentile = Member
top 100th percentile = Casual Player
Anyone with 100 or fewer points = Political Puppet (they end up > 100 percentile).