Speaking of "test not valid on FM", here are the S meter readings I get on each mode on the same 2 meter frequency - actually all acress that band:
FM: S9
AM: S5 to 6
RTTY: S2
CW: S1 to 2
SSB: S4
DV: S6 to 7
I never checked for those variances before and I don't know their significance, if any.