And my results agree with yours.
In semi-auto you can have it set at upper twenties but it may actually be turned way down from there, often much lower than you would like. It might at any given time be where you have it set in semi-auto, or even above what you have it set, thus leading to an occasional deep find. But, If you get into a trashy area or an high EMI area the machine can hugely drop the sensitivity. On the E-Trac you can see where actual sensitivity is, on the earlier models one had to guess. Some people that trusted semi-auto on the explorers will use manual on the Etracs when they see how far the sensitivity can be turned down in auto; probably the exact same thing was happening on the explorers but we had no way of knowing that.
I do believe sensitivity does affect depth. One thing to remember is that varying sensitivity does not vary the strength of the transmitted signal; the whole using your bright lights in fog is complete BS. What sensitivity does is controls how big the signal needs to be in order to be processed. A better analogy would be are you wearing sunglasses, no glasses, a welding hood, or night vision googles? The light from the sun and stars is the same, sensitivity is not a dimmer switch. How bright it appears depends on what you are wearing over your eyes.
I do find a lot of older deeper stuff in the country or in parks away from power lines where I can turn the sensitivity up, as compared to in town under powerlines where running mid teens is often a challenge.
Chris