Meters + Instrument Gain is different than Track Gain

Looking for tips on how to better use the meters, this is the way it's currently setup (to record guitars)
Track 1 mono Guitar Left   75% -----> Guitar bus
Track 2 mono Guitar Right 75% -----> Guitar bus
RECORD IN = -12
PLAYBACK = -18
MIX bus = -6
MASTER bus = -3
 
Also I noticed on an Instrument track, reducing the Gain effects what sounds come through.
(pro channel = off ) Track 3 AD2 Drums stereo  ----->  AD2 bus (keep meter at -18) ?
Instrument track Gain -127 to 127
-127 = no sound
-113 = only snare
-099 = overheads
-087 = Snare, Kick, OH, HH
     0 = Full kit