It is no different than when a roundtrip airline ticket is purchased for travel between the US and UK you get one price if you are purchasing in the US, but another price if your purchase city is in the UK, and still different if your origination city is in the UK, and your round trip is reversed. It is all a matter of competition. All of the travel companies keep track of what their competitors are offering, and partially base their rates/fares on that, assuming that someone from Croatia would not be doing a search on a US site, even though they are picking up the car in Orlando. Odd, I know, but reality nonetheless.