the EFOB in the 'bus is the sum of ALL the fuel the FMC computes you will have at destination.
if you've not yet taken off, EFOB=final reserve + alternate fuel + route reserve + extra fuel.
if you're airborne, EFOB=final reserve + alternate fuel + extra fuel
the difference is not due to airbus but it's jar ops. flight planning makes you take on different quantities at the planning stage (ie taxi + trip fuel + alternate + final reserve + route reserve + extra fuel)
once you're in the air you've only got to manage what fuel you've got on board, provididng you land with at least the final reserve.
so your EFOB doesn't change if you change rte rsrve. what will change is your waiting time (but you'll need to complete the rest of the pages correctly like the right qfu's, winds, etc)
seb