Measured values on my insulation resistance meter vary. What might be causing this?

Q

A

Insulators generally have the following characteristics. Their resistance value varies with temperature. (The resistance value decreases as the temperature increases.) Their resistance value varies with the voltage being used to measure it. (The resistance value decreases as higher voltages are applied during measurement.) Their resistance value decreases as they absorb moisture. (The resistance value decreases dramatically under conditions of high humidity.)