Thanks for playing :D
Thats a great score. The grading and graph is centered around the mean of all scores and range is determined by standard deviation, so at some point with enough data the graphs will plot a nice bell curve.
in simpler terms B grade means you are better than 50%-83.9% of the people currently in the leaderboard, which is above average currently :D
A is >83.9%
S > 97.9%
S+ is only rank 1