Just checked my GTPSP and in comparison to that GT5 picture, these manufacturers/tuners are missing:
AC Cars, Amuse, Alpine, ASL, Bugatti, Bentley, Blitz, Buick, Cadillac, Callaway, Chaparral, Chrysler, Cizeta, DMC, Dome, Eagle, FPV, Gillet, Ginetta, HKS, Hommell, HPA, Hyundai, Jensen Healey, Land Rover, Lister
Yes, theres an arrow pointing to more manufacturers, except the list is in alphabetical order like someone mentioned.
That would suck if even regular dealerships cycle. I have the money, why cant I buy [X] car now!?!??!