The only thing, that makes sense, is they add some kind of a margin for obstacle clearance
The obstacle clearance calculations are separate to the DME/altitude calculations. The advisory altitudes will always remain above the required OCA for any particular approach segment down to the MDA, and may require an increase in the VPA. PANS OPS requires 150m (500ft) obstacle clearance in the intermediate segment and 75m (250ft) in the final segment. This does not preclude an obstacle in the missed approach segment controlling the OCA in the final segment.
VNAV procedures use a different methodology ....
