I think it happened in a mission which restricts you to use Ferrari Enzo. If you choose that mission even restriction message will pop up. Same thing happens in one mission as a racer, which restricts you to use Koenigsegg Agera R. So it is normal just if you want to have all your cars. Then do not play those restricted missions. If you need to complete them, yes you can use only one car. It also happens once more as a cop restricting you to use only one specific cop car. I hope this description is helpful.
