Antenna location makes a great difference; VHF communications are "line-of-sight" and when you are in a truck in front of the terminal building, it will be hard to hear the planes behind the building. There also is the issue of "electric noise"; some types of lamps, computers, other electronics and car ignitions generate noise... placing the receiver antenna strategically away from these sources of noise, silencing the sources of noise, etc. gains you several dB. It is possible that ATC uses transceivers at places several miles from the airport to obtain better communication using cables to connect the signal to the tower
Next there is the issue of antenna quality and receiver quality. Antennas on hand-held scanners are a compromise for portability, connecting a proper full-size antenna makes a lot of difference. At last, professional receivers are more expensive and that is partly because they are more sensitive and have lower internal noise levels. So, it is likely that the controller hears a voice with some noise behind it...
Hmmm, I forgot to mention that controllers are pros and trained to get infornation out of lousy radio connections.
