On the Airbus the Optimum is the most economical flight level, taking into account Cost Index/Wind/OAT
Only 500 ft steps have an economical impact (as far as I know flight levels have 1000 ft steps), and unless you are given a block altitude, 500 steps are not feasible.
So in order to avoid "noising" the FMS i.e. making it computing for a long period of time a "meaningless data" the FMS uses 500 ft increments.
This applies for Honeywell Pegasus, as well as the Thales/Smith FMS2 since they are both developped from Airbus/Aespatial specification.
The 1500 ft is something I am not aware off, however all the above will work only if you are all the time in managed mode, because to the optimum flight level corresponds an optimum path that can only be flown if you are at the optimum speed.
