Test for N-1 by exposing for Zone 9 and developing for a Zone 8 density. Test for N by exposing for Zone 8 and developing for a Zone 8 density. Always blame yourself for inconsistent results.
Exceptions involving the laws of physics are less likely the fault.
It is best to work on overcast days if you are using daylight as the light source. Use the same meter. Use the same lens. Try to use the same shutter speed as much as possible. Always work at the same developer temperature.