I am assuming the reason why this question is asked is because of the relation Setting STD has to the FMA's on the PFD. To state the obvious when flying NAPD you remain in MAN FLX SRS until you have climbed through the NAPD1 altitude you have selected in the Perf page of the FMGC.
There will be some cases with a low QNH that if you were to Set STD whilst still in the NAPD 1 phase, thus climbing via SRS, that setting Standard would cause the aircraft to go from an altitude of 2,700 to FL30 straight away, which is only a QNH change of roughly 10 hPA.
If you selected 3,000 as your ACC altitude in the Perf Page to comply with NAPD 1 then by setting STD, in this case, you will have trigged the aircraft to enter CLIMB mode earlier slightly early, and thus not completing NAPD1, beginning the reduction in nose attitude and the increase in speed.