Hi nophead and ville,
nophead: Ville pointed to the correct spec, the Mooshimeter’s resistance measurement at the low end has a little bit of offset, which is why the accuracy rating only extends from 20Ω upward. The extra resistance you are seeing is a PTC thermistor – a circuit protection element that protects the resistance measurement circuit from overload. This element’s resistance is somewhat temperature dependent. If you hold the probes together and zero your measurement before you measure the next resistance you should see the offset disappear.
Hope this helps, best