I wonder if maybe those muzzle threads aren't as concentric as they could be (just not enough to strike baffles). Only explanation I can think of for why one gun would react so differently,
That was the conventional wisdom until suppressors became so popular, its just impossible to predict, and is only an issue if you plan to shoot suppressed and unsuppressed switching back and forth often, as you just maintain suppressed and unsuppressed zeros -- much as you would for different ammo brands and bullet weights.
My experience is a large shift is the exception, the RDB is only my second example of an annoyingly large shift, the other being my Draco SBR AK.
I move my cans among a variety of guns.
I measure (check) concentricity with a length of precision ground drill rod before shooting a new combination and find little correlation between how well centered it is vs how much POA/POI shift I observe.
In fact I observe the least shift (not detectable in 10 shot groups at 100 yards) with my RRA NM AR rifle, but I debated and repeated my check many times before actually shooting it suppressed.
Here is my thread from when I first got my 5.56 suppressor and started checking:
http://www.thehighroad.org/showthread.php?t=723447
The RDB & RFB gas systems adjust by letting more gas escape (vent) instead of letting less gas through the port, I attribute this as the reason they generally shoot unimpressive groups off sandbags, the RDB doesn't seem quite so bad so far, but I've shot significantly better groups at 50 yards with the RFB kneeling than off a bench with sandbags, I'd have assumed it a fluke but I've done it multiple times.
unless the KT barrel is particularly skinny or long & prone to harmonic effects from the silencer weight.
SUppressor weight is probably the most common reason for a shift, but this will shift low instead of high. It is complicated (harmonics) and it is what it is for any gun/suppressor combination.