Individual a/c's performance is tracked and flight plans adjusted for better, or worse, than standard fuel burn.
Yes, there are 'good' and 'bad' a/c. Time since engine wash, new vs. old engines, drag from damage, are some of the variables.
My last year's fuel burn was approximately 99.5% of expected. Underburned by approx. 0.5% per flight. Standard performance for the fleet.