If it says 95 on the petrol cap, then the ECU only ever tries to run an igniton advance that's appropriate for a 95RON fuel. If you put anything higher octane in there, then the ECU has no way of either knowing or advancing the ignition timing any further.
If it says 98 on the cap, then the ECU tries to run an ignition advance that's appropriate for a 98RON fuel. If you then put in 95 RON fuel, the engine will start knocking, the knock sensors will detect it, and the ECU will retard the timing advance enough to prevent damage to the engine. With less advance you get less power.