This might be from induction caused by current flowing to close to the antenna in your phone. This is probably more noticeable when you have lower signal (1-2 bars) as your signal may drop due to the wire being to close to the phone or the internal circuitry is to close to the antenna causing inductive interference on your signal.
You're probably right in your analysis. Then again, I've never had any signal drop with my iPhone 4 and 30-pin cable.