I was under the impression that dbm was a measurement of losses.
So a perfect signal (lets say you are standing on top of the Verizon Tower) would be (0). There is no such thing as positive losses, and anyone can write on Wiki, that doesn't prove anything. ;-)
I know when we periodically test our fiber-optic cables, they put a known light source on one end, and then measure the light at the other end and the result is a negative dbm value. And that value represents the loss of light over the length of the cable. I know this is apples and oranges, but I think RF signals work the same way. They are broadcasting a known RF level, and the level being recieved by your antenna on your phone minus the original level being broadcast equals your dbm loss, and it's always negative. Even a zero signal isn't really possible, only hypothetically, but a positive value is not even hypothetically possible.