By design... it scales the power of the cell radio so as to conserve battery power when you're not in a call... It has full reception, but it doesn't need it, so it power-saves on the cell radio to just have enough signal to get a lock on the tower, then when you get or make a call, send a text, etc, then it powers up and gets full reception...
At least I think that's what's happening, since mine does it too.