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Many factors can affect a reading. Haze or cloud will produce more light pollution. Early evening readings tend to be a bit worse than late night or early morning ones when more lights go off in the suburbs. I have an oval nearby with pretty fierce flood lights.
The Milky Way at zenith can make a big difference. Agree with John on the difference. Pointing away from the Milky Way is better but the light pollution reading is worse at lower elevations. I would take the readings when the Milky Way is lowest in the sky.
With several measurements over a period of time, you should get some consistency. At my place, it was about 20.2 10 years ago but seems to be heading to 20.1 or worse due to light pollution. Hence, the need to head out further from Sydney to get darker sky.
Regards, Rob
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