At Hertz, YB is normally an enhanced and sporty version of a B sized car, so it could have been considered an upgrade from a regular C class vehicle. However, you should have been asked if you wanted such a free upgrade. You should not have been charged more than the amount in the reservation unless you had requested the upgrade and agreed to pay extra for it. Since YB is smaller even if it's a more expensive vehicle, you should been asked if you were willing to accept the substitution (for free) and not just given it, perhaps in hopes that you would not notice.

I like YB vehicles. What car did you ultimately get as your sporty compact?