Sound travel is a strange one, I think its all to do with the frequency and probably the weather conditions will have some effect as well. You can have one bellbox that's ear piercing loud close the property but can barely be heard down the road, then you can have an other sounder that's not quite as loud but can be heard down the road, Generally alarm sounders range from 1 to 3hz, but unfortunately when you buy a bellbox it only tells you the db rating not the frequency, the tone of the siren may have some effect on how far it travels whether its fast or more of an ambulance warble type.
One thing I have noticed is the keypad entry/exit bleepers they can be heard from quite a distance away from a property, (at least on mine anyway) and they are only 80 odd decibels, nowhere near as loud as a bellbox but can be heard at almost the same distance as the bellbox which is rated around 115db?
I think its all to do with the frequency and the weather combined at least that's my take on it, I'm no expert, If someone can explain better then by all means.