Hy LEM :
My guess the explanation is this:
from the nnc 14.6 'ldg gear lever jammed in up position'....
' Set Vref 15 +10 kts.
[ Provides tail clearance during landing since hydraulic system A flight spoilers will be unlocked and may float] '
what happens is this: with ldg gear lever jammed in up position the hydraulic pressure is locked in system A,that's why you have to use the speed braake lever to relieve pressure from sys A in order to extend the ldg gear.
By doing this,you'll have no pressure in sys A,and the inboard flight spoilers will be not only inoperative,as in the other nn case-loss of press sys A-,but more important ,they'll be UNLOCKED,so a chance to deploy when landing,causing a tail strike.
That's the reason for flap 15 ldg in this case,not necessary though in the other case-loss of sys A-,where the flight spoilers will remain inoperative but locked.
Then again,that's my guess,but I think I'm right.
Brgds Alex