When operating at the most sensitive input voltage ranges, there may be a small offset (perhaps a few millivolts) on the voltage readings.
Go to Settings | Offset null and choose the channel you desire for compensation.
To remove this offset, short the positive and negative leads together. PicoScope measures the voltage on the channel and then removes this offset. The offset is not stored, so it is necessary to repeat this procedure each time you start PicoScope if you need to compensate again.
I hope this answers your question.