I seem to have a very poor phone signal in my house and if I am in any of the buildings at work. They are 20 mins drive from each other. I have an Samsung Galaxy S4.My coworker has an iphone and has no problems and my wife has an S2 on a different network and has no problem at home.
That she is on a different provider than you.. quite possibly answers the question at least partially. Signals, only go so far, etc... and THINGS in the way, buildings, etc, can lessen the signal. Likely her carrier, may have a tower which is partially closer to you, to the signal is that much stronger, and gets picked up better there.
The difference between phones, could make a difference too. the antenna in one, might be different than in the next.. so one might pick up a signal better in one area, than another.
HOPEFULLY soon, as they roll out the 700mhz spectrum... it actually is a FARTHER reaching signal, so may provide some more in some areas.