21 over 3 means 7 difficulty increases. Each difficulty increase is by 2 points of bizarre so 7 times 2 for a difficulty of 14. 10 bizarre is 4 less than 14 which means you have 40% less chance to succeed the check. The odds when you have the same level stat as the challenge is 50% so 50%-40% = 10% exact;y as you got.