My setup is Hustler MO-2 with center loading coils and top stinger [appropriate
for each band].

For the sake of this discussion I'll refer to my 60 meter antenna. The antenna
is tuned to resonance at 5.3715 MHz using an MFJ-259B. Adding approximately 4
inches [small spring for coil strain release] immediately below the coil and at
the top of the MO-2 has negligible effect on resonance [a few KHz]. Now, varying
the stinger above the loading coil by 4 inches causes very large changes in
resonant frequency of over 300 KHz.

What's happening in the world of Antenna Physics to account for the change in
sensitivity as a function of position above or below the loading coil?

Having asked the question, I'll slide back into the internet ether, wait for
sunspots to improve, and read the opinions and comments that follow.