That's beacuase in real basketball players can loose (or gain) confidence, consequentl y it can be a good idea to put in a worse sub because in that particlare game he is better then the starter, due to the starters lost confidence.
Now, a BB-play er can miss ten consecutive shots, but this will still no affect his confidence (a s this is not modelled in BB), therefore the likelihood that he will score on the next shot is still the same as before he missed, i.e. the starter is still bett er than the sub.