This change was introduced with the Car Prize Cache update for Andatura 4.
Details
The game will now automatically maintain Limited Edition Cars (besides the ones I choose for Holidays). Currently 5% of production cars will be automatically set to be Limited Edition. The game will randomly choose an out of production vehicle when this happens and is integrated into the Car Prize Cache Forum posts(You can see this currently in the Car Discussion Forum) and the list of cars integrated into the system. As cars trickle in through user car submissions, new Limited Edition vehicles will be selected to maintain a balance if at anytime it drops below 5%.