Hi there,
You're probably operating a 'mixed' fleet of A320's with both the new and old alternate braking system.
The new alternate brake-system employs an ABCU which will modulate the pressure to 1000 psi. The old system does not, so will give you up to 3000psi (if the accumulator is full) during the check. The new system does not require the alternate brake check but is probably still done where there are 'mixed' fleets for SOP harmonisation.
edit: I must add that alternate braking is limited to 1000 psi on alternate braking by the ABCU, only when anti-skid is lost (BSCU lost or off). During the check the NWS switch is off, so will be the BSCU.