High Efficiency Speaker Asylum: Horn driver displacement below FL by pooge

Does anyone know why, or for what relevant specs of a driver, the driver displacement drops below electrical resonance at FL, yet remains high (i.e., at the same displacement as at FL) at freqencies below FL. As an illustration, compare the McCauley 6342 8ohm version vs. its 4ohm version. In the 8 ohm version, displacement drops below electrical resonance at FL, but in the 4ohm version, it doesn't drop. (This is in a McBean model, 80-500Hz, t=.6.) Thedriver specs are at www.mccauley.com. The BL factor is only listed for 8 ohms. I took it to be the same for 4 ohms:

  
   Bl Factor 25.3 T*m(8)  
   Moving Mass 0.099 kg  
      
   Fs Free Air Resonance 40 Hz  
   Re D.C. Coil Resistance 3.4(4) 6.0(8) 11.5(16) ohms  
   Qts Total Quality Factor 0.20  
   Qms Mechanical Losses 7.40  
   Qes Electrical Losses 0.21  
   Vas Equivalent Volume of Air 200 L / 7.07 ft3  
   Sd Effective Piston Area .089 m2 / 138 in2  
   Xmax Excursion - Max Bl 4.6 mm / 0.18  
   Vd Sd x Xmax 411 cm3 / 25.1 in3  
   Le Inductance of V.C. 0.6 mH (4) 1.1 mH(8) 2.2mH(16)