The reason you only fill your Instant Pot 2/3 of the way is because that extra space is needed for the fluids that will be released from the contents as they are under pressure and/or for room for grains to expand as they cook.
The reason you only fill your Instant Pot 2/3 of the way is because that extra space is needed for the fluids that will be released from the contents as they are under pressure and/or for room for grains to expand as they cook.