it just my guess. I think due to Flaps 15 usually used as flap position for one engine inop. For more reliable work of last engine under icing condition.
Hi Felix! It might be due to the fact that VREF ICE is associated to conditions when the stick shaker logic would be different (activation at a lower AoA), ex anti ice operation during approach and landing or wing anti ice has been used at any time during the flight... With Flaps 15 your stall speed would be higher compared to Flaps 30 or 40 and the aircraft would descent on the glide with a higher AoA, combined with just one engine providing thrust i assume that the extra speed is a safety margin to stall speed for both approach and go around.