[Elecraft] P3 Reference Level

[N7FN at comcast.net]

I would expect to adjust my P3 REF LVL to S0 and see the noise and/or 
signals displayed at various "S" levels above S0.
(K3 set up for JT-65, CUT: 0-2.4, AGC-S, TX DATA, XFIL 2.8)
However, when I adjust my REF LVL to S0 with the appropriate contrast on the 
bottom screen, on:
160m the REF LVL is about 3 S-units (-S3) below S0
80m  the REF LVL is at S0 as expected
40m the REF LVL is at S0 as expected
20m the REF LVL has to be adjusted down about 1 S-unit to get the 
appropriate contrast on the bottom screen
15m after readjusting the REF LVL for S0, the Ref Level jumps down 3 S-units
10m the REF LVL jumps down 4 S-units.

Why can't I adjust the REF LVL for S0 and get the appropriate contrast on 
the bottom screen without adjustment on each band?
I realize that the contrast may vary with the noise on the frequency but I 
think that S0 should be the same on each band selected.

What am I missing here?
Is this normal operation?
Am I missing an adjustment?

Frank
N7FN