Right - so having looked at a couple of online (probably obsolete) FCTM documents:
A330 A340 Flight Crew Training Manual
(p. 2.60.5)
http://www.737ng.co.uk/A320%20321%20...g%20Manual.pdf
(p. 10-12)
The first document relates to the A330/340 and states:
In level flight, if the A/THR is disengaged and thrust set to idle, the aircraft decelerates until the auto-trim stops. This occurs at a predetermined angle of attack called Alpha Prot. The speed that equates to Alpha Prot (Va PROT) is displayed as the top of a black and amber strip on the PFD speed scale. If no input is made on the sidestick, the aircraft will descend to maintain its current AOA (Va PROT). To maintain the flight path, the pilot must increase the backpressure on the sidestick, which also provides a tactile indication that auto-trim has stopped. At Va PROT, AOA protection becomes active and, if the sidestick is released to neutral and no thrust applied, the aircraft will gently descend maintaining Va PROT. When AOA protection is active, the speed brakes retract automatically, if previously extended, and the bank angle limit is reduced from 67° to 45°.
If the pilot maintains the backpressure, Alpha Floor (covered below) will activate. If the pilot disconnects the A/THR while maintaining full back stick, Alpha Max may be reached. The speed which equates to Alpha Max (Va MAX) is displayed as the top of the red strip on the PFD speed scale. Alpha Max is close to, but short of the 1g stall. When flying at Va MAX, the pilot can make gentle turns if necessary. In turbulence, airspeed may fall temporarily below Va MAX without significant effect.
So what the FCTM is saying is that when holding full back stick in High AoA Protection mode Alpha Max *may* be reached, but it says nothing about the aircraft being held there precisely.
The second (A320 series) doc states:
• The PF will notice if the normal flight envelope is exceeded for any reason, because the autopitch trim will stop, the aircraft will sink to maintain its current AOA (alpha PROT, strong static stability), and a significant change in aircraft behavior will occur.
• If the PF then pulls the sidestick full aft, a maximum AOA (approximately corresponding to CL Max) is commanded. In addition, the speedbrakes will automatically retract, if extended.
...
• HIGH AOA protection: Provides maximum aerodynamic lift
So it also makes no claim that Alpha Max (as illustrated on the graph you posted) will be held precisely.
Both documents indicate that the system will hold the AoA at a level calculated to provide maximum lift, but given the constantly shifting variables behind that calculation it makes sense that the value will actually lie somewhere between Alpha Prot and Alpha Max (the range indicated on the PFD by alternating amber lines on the speed tape) while the stick is held aft. Releasing the stick commands the aircraft to maintain Alpha Prot.
Of course, while these documents are probably obsolete, they've undoubtedly changed somewhat since 1988, so you'd need data of that vintage to be sure.
[EDIT : I think I've actually learned stuff today!
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