You found a nice hire car in Bulgaria on some rent a car site. You like the model, the make, the year of manufacture as well as the price and decide to go for it and make a reservation. On the first day of the reservation you go to the rent a car company to pick up your rental car and... oh, surprise. The rental car is not the car you ordered. Neither the year of manufacture, nor the model, nor the make match. The only thing matching your reservation is the price as well as the rental car's size. You are most probably disappointed if you wanted to rent and enjoy the new VW Golf and ended up driving a 10-year old Fiat Brava.
This of course is a very grim scenario, but it describes some of the bad practices, you might fall victim to when hiring a rental car. And when you ask the car rental guy about it, he'll most probably answer with a smile on his face: "...on our site it says VW Golf or similar..." So here is a piece of advice from me - ask about the specifics of the hire car during the reservation and if the car of your choice is not available, ask about the so called "similar car". If it does not suit you, you can go to some other car rental in Bulgaria, make the desired reservation and avoid being forced to rent a car you don't want to in the last minute.