Those carrier coverage map programs are not all they are cracked up to be.
A lot of them do a speed test.. and they often use a server out there that is not dedicated or overloaded and that makes the speed test portion of it irrelevant.
That's the first bit of it that is just awfully wrong and not anything I would even consider.
The next issue with these apps is that a lot of them test in the background and it's impossible for the app to take into consideration actual use.
The phone could be in a console in your car and as a result the signal is attenuated by it's surroundings.
I have only seen one of these apps that had a sensible approach to this issue by having a mode that did it's tests while the device was in use.
Testing while it's in use is more likely to be accurate because it's probably in the hands of the user and not stuffed into pockets, purses, consoles, etc.
I have found by comparison that the carriers coverage maps are actually more accurate than these carrier coverage maps.
Take them with a grain of salt... a really big grain of salt.
I have found them to be grossly inaccurate on the east coast in a lot of metro areas.