I say again .. where in FCOM does it say this?
Check FCOM 1.22.30 P10
Another good reason for using Selected speed is .. on a go-around the a/c will not accelerate automatically at Aa but will remain at the pilot-selected speed.
If you insert a speed constraint in the missed approach routing the aircraft will accelerate to that speed, so you can keep speed managed, i.e you need to fly 175 kt for go around, then insert 175 kt as a speed costraint. If you pull speed during the go around you exit the SRS and you go in OP CLB.
Cheers.