Ok, I think I know what the issue might be. Here's what I did;
1. put a signal generator on a mono track insert 1, sine wave -20dBFS
2. route the output of the above to a mono group track
The signal now reads lower on the meters at the bottom of the actual group track. On the audio track it reads -20dBFS, and on the group it reads -23dBFS. I then looked at the meter on the group track and noticed it consists of two meters which made me think that it has to do with where it measures... or something.... so;
3. created another mono group and routed the first group to the second, and voila! The second now reads -20 and the third (new group) reads -23. The meter is now also single in the first group track and double in the second.
4. put the SLM128 in the last slots to confirm measurement, -20dBFS on all tracks/groups.
Now it appears that this surely is an issue with where the signal is being measured, so I right clicked the panner section and went to "Global Meter Settings" and changed it from "Meter Post-Panner" to "Meter Post-Fader" and sure enough I now get one "bar" instead of two and it reads correctly.
That's why I kept asking you where you measured the signal and how.
Please PM for conversation.