Measuring return air wet bulb temperatures will make the difference. Note how wet bulb affects superheat in the chart below when the outdoor dry bulb (ambient) temperature remains the same but the indoor wet bulb temperature varies:
Other charts are out there that show how the indoor coil split can vary according to indoor return air wet bulb temperature. If conditions are right, you can see an indoor split as low as 15 degrees and the system is properly charged and working correctly. That would be typically very humid air and a higher than normal return air dry bulb temperature, but nevertheless it is possible.
Building Physics Rule #1: Hot flows to cold.
Building Physics Rule #2: Higher air pressure moves toward lower air pressure
Building Physics Rule #3: Higher moisture concentration moves toward lower moisture concentration.