My assumption is that trash mobs are dropping more items at higher MP, but those items tend to be lower iLvl. So you may be getting more items, but more of them are junk also. (Exclude Legendaries from this hypothesis)
And you are seeing less i63 because more of those come from Elites which you kill less of at higher MP.
In conclusion, my hypothetical assumption is... Trash mobs dropping more trash items at higher MP, but less Elites dropping less high iLvl items due to slower kill speed and lower encounter rate.
Whereas on MP1 you kill way more elites, seeing higher drops of iLV63 items... but are getting less item drops from the trash monsters so your IPH reflects that.