In managed the aircraft will target S or F depending on flap lever selection. With a slat flap fault you have failed slats and or flap so you will not get the same coefficient of lift therefore you need a higher speed to be safe when you select the next stage at VFE Next -5. A stall warning may occur when flap goes down with no slat if you are too slow. When landing with a reduced slat/flap angle you have less drag so you do not want G/S mini to interfere with the approach speed in case you land too fast and don't have enough stopping distance on a short RW. Hence selected speed at, for example, VRef +25 in the case of dual slat system failure. Also in the latter case you will have lost all auto-flight functions and characteristic speeds on the PFD - no GRN Dot, S, F, VAPP or VLS so you can only fly selected speed.
Does that help?