What is the reason for these strange radio altimeter calls? I ask as a dilettante, so please feel free to correct or emend this. 

I transcribe the radio altimeter enunciations for https://www.youtube.com/watch?v=Vr-0LiQA6wo:


  0m 21s: 400
  
  0m 23s: 400
  
  0m 36s: 100 above
  
  0m 41s: 200
  
  0m 49s: 100



Are the radio altimeter verbalisations part of the Ground Proximity Warning System? 
Why was 400 uttered twice? Isn't the A380 descending?
Same question for 100 at 49 s, compared with 36 s? 
What does the above imply?
"Four hundred" was likely annunciated twice due to a bit of a dip in the terrain that would cause it to pass the 400ft level twice. The "hundred above" call-out is for 100 feet above decision height to let the crew know they will need to make their decision soon to either continue or go around. After that, you hear the "two hundred" and "minimum" call-outs right after one another. The "one hundred" call-out afterwards signifies 100 feet above ground level.