As far as I know "ability" is just as you said, how good a jumper is, so how long he can jump....
the precentage of "security" I think influences how often a player can show his full "ability" ...
So in an example
Player A:
Ability 100
Security 0
Player B:
Ability 0
Security 100
Player A will do one jump on extreme high level and the next on a very poor level, he will jump complete inconstant...
Player B on the other side should jump very constant as he has high security, so he will do almost the same jump every time... But as he has Ability 0, he will jump short all the time...
Hope my explanation is first correct and second understandable...
Greets