APU GEN has no CSD
My best guess is that the APU GEN has no constant speed drive like the two engine generators.
My company asked Airbus some time ago if they recommend to start the APU before commencing a low visibility approach (it has been a common practise by commanders who flew the MD before)
The answer was more or less: "as you like but there is no need for it and doing so doesn't improve your chances against a downgrading because the APU GEN has no constant speed drive."
W.