that's simply what it says in the SOPs
as to the reasoning, obviously there is no aerodynamic influence of the elevator position from a standing start, I'll give you that
it really doesn't appear to be entirely logical, and I can only surmise it is more logical for a rolling take-off where the power is set at a higher speed where there will then be some tailplane effectiveness.
as to what the speed is where it has an influence, Airbus do not publish that with the manual set.