When you have time, I would like to get some more data points.
We know the meter doesn't show open circuit, but does show 0.0 when the probes are shorted. If you have a decade resistor box or some resistors, I would like to try one resistor for each range.
For example, measure a 100 ohm resistor in the 400 ohm range, a 1k ohm resistor in the 4k ohm range, etc. It doesn't have to be 100 ohm, just something between 1 and 399 ohm.