Unless you have a terrible wires for ground, it should have very low-impedance.
But not completely impossible:
3.3V throught4.7K draws 702microamps, if the ground wire+connector adds 30mOhms, it would drop 22uV, just enough for 1ÂșC offset.
Your station reads 12 when the switch is closed.
ADC 12 = 9.67mV, amplified x250, so input is 38uV.
I remember the chinese JBC handle having pretty thin wires.
My quicko has 2 wires and 2 pins in the GX-12 connector for gnd
Try replacing the 4.7K pullup with 47-100K and see what happens.
But that doesn't explain the strange behavior in Rick's station. It reads low.