Usually more stars means that you did better or that you did it in a more difficult more. However in this game each truck you add gives you more stars, so if you complete, for example, a map with 7 stars it means that you did choose the easiest setup INSTEAD of the hardest, which would be currently 1 star (a single 469).
My suggestion is that 7 stars should be starting with an uaz only.
Its nothing important... But this game is filled with "nothing important" small annoying things that could be polished, and tbh it wouldnt cost much fixing them.