If your SLM is a type 3 (survey) instrument, +/- 6 dB is within the error band of the device. If type 2, you should not see more than 4 dB difference.

Could be differences in microphone placement when you change tones, assuming you might move the SLM. You could see standing wave, destructive or constructive interferences.

If your SLM is reading in C scale and not true flat response, the difference could be due to a difference in low frequency energy between the test tones since the C weighting scale is not flat. Dumb response here. C and Flat shouldn't matter if the test tones are different only in low frequency energy. Each will detect a difference using the same instrument and weighting.

Just some guesses.

Del