mono LP. record in stereo, convert to mono

I don't remember making measurements on most LP recordings I ‘ve converted to mono, but I know I have found the -/+ 3dB difference I expected on at least some of them: a 3dB decrease in background noise, an approximate 3dB increase in signal level. With cassette source there has been the decrease in noise but no increase in music volume.

I can only assume this current project is monaural, due to the waveform appearance and the sound I hear in my headphones. There isn't a word of English anywhere on it, but I'm fairly sure what is there doesn't say anything about stereo or mono.

I made measurements because I noticed that the between tracks noise level seemed to have really dropped after the conversion. Measuring several places between different tracks, I found the decrease to be about 9.6dB. The conversion to monaural was done after extensive declicking.

Then I went back to the pre-declicking version and copied out a bit more than one track, this one the last song on side one. I made the following measurements with CE's Analyze/Statistics. The conversion to mono was made with the Channel Mixer's LR to Mid-Side preset so that I got both the sum and difference on screen. The left hand column is the sum (the good result), the right hand column the throw-away difference.

before music
Min -09.08 -3.27
Max -12.67 -0.96
Avg -11.24 -1.12

2:20 of music
Min +0.10 -16.67
Max +0.58 -18.78
Avg +0.63 -19.7

The decrease in noise level is three to four times what I though was available through this process and the increase in the signal level seems negligible. Do these numbers indicate something other than a monophonic recording played with a stereo cartridge or have I just had too limited an idea of how things work?