They are measuring voltage, not resistance (the resistance is computed). It's impossible to measure resistance without current and some voltage across the resistance, and while the parasitic effects are small the resistance we're talking about (and hence the voltage across that resistance) is so small that second order effects introduced by the probe wires can have a real effect, for instance if the probe wires have a higher resistance that makes them more susceptible to electric fields, which in turn would show up as a voltage. So this is anything but trivial.