Farid,
This is just the way it is presented on the PERF page on the FMGC with respect to the Airbus. When flying a CAT 1 ILS the DA goes in the MDA field. When you are flying a CAT 2 or CAT 3 approach just below it there is a DH field, where you put in the RAD ALT DH. What ever the pilot inputs into this field, it will appear on the pilot primary flight display. It also drives the synthetic voice for the pilot i.e. '100 above' and 'Minimums' etc.
Hope this helps.