Alright Dan, I’ll give it a shot here to what is happening.
First and foremost if one wants to look for deep nickes 5khz single freq is not where the action will be.
Better to use park 2, field 2 or prospecting modes if one has 800 model.
As far as what you were witnessing. Single freq ops using 5khz is not as resonant on lower conduciors hence Nox won’t hit as strong and or will have tendency to give ID in error. You were seeing up averaging of signal. Remember tones wise, disc wise or notch wise Nox tone and ID is based off of how the detector feels about the target. Hence you had the junk disc or notched out yet it gave tone and ID ( in error).
An Eqx user could witness what you dId using other freqs available on Eqx using single freq ops. ID of targets solo buried multi frequency will give more accurate tone and ID to boot.
My experiences show the use of 5 kHz single freq can have benefits looking for higher conductors in modern( lower conductive trash). Expect what you find higher conductive wise located with use of 5 kHz that was missed with mulit freq to to lean more on the shallower side depth wise.
As a target masked (higher conductive) gets deeper below lower conductive junk mulit freq will have equal or even better advantage in seeing the higher conductor.
And naturally using 5khz one has to be mindful of the good ole bottlecaps, as they will read high like a higher conducive coin where transitioning to multifreq will generally show a lot lower ID exposing.