Hi,
Ok, this problem might have something to do with the CURRENT levels we are using. Recall that some switches made for higher currents do not work well for lower currents and voltages, and the same for some relays. This is why mercury wetted relay contacts came into existence (instrumentation). Relays like this will have a low current rating as well as a high current rating.
So keeping that phenomenon in mind, i believe this is what we are seeing here. My meter probably only puts out 1ma for the test or maybe even less. Some meters will put out more and so might measure the contact resistance for a *higher current load*.
Of course this doesnt rule out other possibly explanations too, but at least we can be sure this is one of them. But to be really sure i can test the same boards with a higher current level and see what gives. I am betting that there is a threshold that once reached, shows lower resistance, just like some switches do.
I once measured a switch that was installed in an electrical box with an ohm meter (standard 1ma or so test current) and it measured completely OPEN. However, turning the switch ON when in the lamp application caused the light to turn on normally. The voltage potential is higher for the 120vac line than for the 3v ohm meter so that is a possible explanation for that too. The higher voltage establishes a 'connection' whereas the lower voltage cant get through the oxide, even though it can be just a tiny amount.
YES, cleaning the contacts might help, but i dont think i want to spray the board with contact cleaner and have it sitting inside the contacts for a while. Maybe smacking it a few times to remove the bulk of the fluid though. I'll have to give this some thought.