thanks for clearing that up because i dont understand car sales at all..
monte carlo vs thunderbird is probably because in the 70's and early 80s they were comparable. the fact they compared a w body car to the fox body was probably a battle of what they've become more than actually having the cars in the same class.
in car "sales" chevy luxury < buick luxury < cadillac luxury, and cadillac tries to stand alone as the best "luxury" american car. until recently Cadillac rarely took on bmw, audi, jaguar, etc directly. So to say that a chevy is in the same class as a bmw is like saying an aveo competes with a gti.