Visual approach/landing:
You shouldn't hear the RETARD - if you do it is to tell you that you should of selected idle power. I select idle at 30ft and never hear RETARD. Works well. If it is not done by 20ft then you get RETARD. It is a reminder to allow speed to decrease below Vapp and not go into speed reversionary mode at the flare!!
Automatic landing:
You do hear the retard call at 20ft, it is an instruction. The engines have already commanded THR IDLE by now but for same reasons as above, you have to physically select Thrust Levers to IDLE.
Remember on the 'bus, the TL's are switches not throttles connected by cables to the donks.