i knew i had seen that info somewhere, and here it is:
when you go to the 'high scores' page, click on 'monthly competitions' and then on 'success rate competition', here are the details shown there:
In the success-rate challenge, players compete to achieve the highest success-rate percentage for puzzles solved vs. puzzles played. Only players who are ranked in the top 50 of that month's high-score challenge are eligible for this contest. (Ties are broken by total accumulated points.)
and for the 'fastest solver competition':
"In the fastest-solver challenge, players compete to achieve the lowest average solving time for all puzzles solved during the competition month. Only players who are ranked in the top 50 of that month's high-score challenge, and who have a solving percentage of 75% or better, are eligible for this contest."