Out of curiosity, I looked up Medaka first week sales starting from volume 10 through the current (vol 15, which was last week), and it's interesting: they go 104k - 92k - 105k - 71k - 69k - 103k. I wonder what caused the drop for vols. 13-14?
Anyway, more relevant to anime discussion (since I doubt volume 15 was affected by the anime at all), volume 16 probably won't be out for a bit, so we'll have to see what happens then.
Some times they get less days to sell... like from the list you posted.... (just guessing the amount of days...)
104k - 4 days
92k - 4 days
105k - 4 days
71k - 3 days
69k - 3 days
103k - 4 days
On another helpful forum... sometimes you see Naruto or Bleach sell really low and someone always comes in the sales thread and goes lol and than someone posts "was on-sale for 3 days only) etc. Seeing as how Medaka sold over 100k and AnE did 520k and HxH did 600k+... I'm guessing this was a normal week for releases. Medaka really does need a bump from the anime though.. sales have been pretty stagnant of late.