A few thoughts:
- Were CGs the same? Aft CG can give substantially lower stalling speeds than forward CG, particularly if the aeroplane is such that at aft CG you are reaching stall before full back stick, but at forward CG you are getting a "mush" at full back stick.
- How good were your low speed calibrations on your airspeed measurement systems? Down to around 1.1Vs calibration tends to be fairly straightforward, below that can be a real headache, and significant nonlinearities in PEC are likely.
- Were they being trimmed identically? I have seen significant stall effects that came down to how the pilot trimmed the aeroplane prior to a stall test, and the mechanisation of the elevator trim tab.
G