About 3 years ago I clocked some Win ST's out of a 5" EAA and a S&W 1006 and they were mid or upper 1200FPS range, but in my G29 were mid 1100's, and I was really surprised in the difference. The S&W had slightly higher speed about 30 to 50 FPS and we put it down to new EAA and used Smith.
The G20 only has a 4.25" barrel best I remember, and test weapon probably was 5" "test barrel", other words not an automatic or revolver. In fact the "test barrel" would be better compared to using a T/C G2 Contender 6.5" barrel model for test equal. Sure would account fo difference in speed reported and mfg stated velocity.
I bet my 10" barrel 10mm T/C would clock Win ST closer to 1325/1350 range.