> It would help to distinguish between (1) antenna detuning because of bridging between the antennas and (2) attenuation because of having a lump of human body next to the antenna.

It is, without doubt, #1. Any confusion stems from the variety of ways in which it can fail to happen (variable conductivity of people, for one), or fail to have dramatic results (being in an area with strong signal).

If you're sufficiently conductive, there's a good chance you can reproduce the problem by walking into an Apple store and trying your hand. That's what I did, and there was a dramatic difference between bridging both antennas with just an inch of finger vs covering the left antenna with as much palm as possible. That wiped out any remaining skepticism I might have had.