The point is probability!
A 2% chance to fumble doesn't mean never.
Every single play is a series of random dice rolls so you can ha e the advantage by roster and play call but a bad roll can negate that.
Luck is the factor nobody wants to accept but it plays a part in games.
A 100 avoid fumble player will fumble less then a 50 avoid player over his career but when the 100 avoid fumble player fumbles in a game...it feels like BS.
Also remember, the defensive player causing the strip ball may be rated high, fatigue and injury can negatively effect avoid fumble....there are more then 1 variable for it all.
It's like saying how can my 100 pass accuracy QB throw an incompletion or my 100 kick accuracy Kicker miss a 40 yard feildgoal. The 100 rating is just 1 variable in a bigger formula to create outcomes based on random number generation.
Hope that helps