This one just drops the nose slightly and it enters into a kind of a "nodding" (pitch oscillation) motion.
I had a Cessna 206, with a Robertson STOL kit do this repeatedly during flight testing. Throuble was that in the test configuration, it did not ever reach the aft pitch control stop (though I did reach the forward stop several times - but that's a different tale!). Therefore, with the nodding going on, and a 10 knot speed variation, I could not pin down one particular stall speed.
I had the opportunity to refly the aircraft with a regulatory authority test pilot, and he made the same observation. He explained that for the purpose of certification, the speed at which the nodding began (being the higher speed in the range of nodding) would be the stall speed, as that was the first speed at which an uncontrollable pitching down began. Flying more slowly, though sometimes possible, was not required, to demonstrate design compliance.
A 2 degree trim tab difference would indicate to me that there are other differences between the two aircraft too (all other things beign equal I presume). I have this with two different Piper Navajos, I have been flight testing recently. Quite different trim tab posititons, for otherwise similar configurations. I'm not far enough into the flight testing yet to compare stall speeds between them though.
On another stall test program years ago, I found that the factory new aircraft I was flying, though a correct stall speed per the flight manual, had wild spin tendancies during a very carefully entered stall. Other aircraft produced in the same group did not. The reason went undiscovered for some time, and was later found to be manufacturing variation, which resulted in the leading edge skin installation (and hence the airfoil) varying from aircraft to aircraft (and in the case of this aircraft - wing to wing). Once corrected, the aircraft are delightful to stall.
My experience is that small factors, including so many mentioned here, can affect stall speeds. You might find that by triming one of the test aircraft in pitch, so as to be well out of trim, you might affect the tailplane effectiveness, and thus stall speed. Though not useful for showing design compliance, if you get the two aircraft to stall the same way, with different trim (or other control positions) it might offer you some clues as to where to look next.
Good luck...