I wonder if the car that best fits the constraints is always the same (i.e : do we see the same car, or each user sees a different car) ?
Only one car would best fit the constraints (that's by definition). However, it's probably not the car you are seeing, because the algorithm tends to get stuck on local maxima. After about 2 hours of running, the thing has flatlined as each window I opened has 20 copies of pretty much the same car (although they differ between windows), and they are not getting that far.