Hi again Dave,
Look and see if the Nano has an option to display the reactive part as ohms instead of capacitor/inductor units.
That'll make it much easier to interpret.
For example - in your 30 m measurements.... at 10 MHz, 402 nH is +25 ohms, while 572 pF is -27 ohms. So your 30 m results show that you are actually very close to one of those zero crossings. The change in counterpoise just shifts the zero crossing a little bit either way. That's not as apparent with nH and pF.