StickyB
Like you I am guessing here but I suspect the reason that the cell base stations and rooftop pager transmitters are (hopefull!) several hundred feet away even if you are flying right over the top of them.
The received signal strenght from a transmitter diminishes as the SQUARE of the distance. So even a relatively powerful cell transmitter would be quite a weak signal by the time it reaches the aircraft flying overhead.
Your cellphone, stowed in your briefcase in the overhead, could be within inches of the wiring between the flight deck and aircraft controls at the rear (the quite important ones that make the aircraft go up an down!!)
As I said earlier, pax really must be made aware that "off" means "OFF" !