The OP's write up is informative (even though I can't see his images), but seems to be missing a crucial piece of information that is missed by subsequent commenters. The OP says he "tunes for field strength"; O.K., that would be like making a 1/2λ vertical antenna a little longer, say to 5/8λ, because a 5/8λ antenna has greater field strength than a 1/2λ, or 1/4λ ground plane antenna, at the horizon, and it does, but even though the 5/8λ antenna is naturally not a resonant length, and if I remember correctly, it has an impedance & reactance of (75-425j)Ω, a good antenna designer, or radio operator, knows they have to bring that antenna into an impedance match with the rest of the radio system, typically 50 Ohms, otherwise they're gain at the horizon will be totally lost because of the impedance mismatch.
Another example is an end-fed half wave antenna which has a feed point impedance of between 2450-3600 Ohms, and is a resonant length, even though in a 50 Ohm system, your SWR is over 50:1. So, don't confuse resonance with impedance and reactance. Any resonant or non-resonant antenna length, can be brought into a perfect impedance match with the radio and coax, and must be matched for the optimal power transfer. The fact that the OP achieves a more desirable radiation pattern by his method of tuning is great, but after getting the pattern he wants, he can then use his Nano-VNA to figure out how to get a perfect impedance match to the rest of his system, by adding series or parallel capacitance or inductance, and/or a tuning stub.
Folks who tune for the lowest SWR are certainly doing the right thing if that is the only instrument you have, but for someone as serious as the OP, he's on the right track in optimizing his pattern for the effect that he wants.
