I noticed that AMR probably calculate amount of mana in our rotation. Sometimes on longer encounters better is to have more mana than stronger spells to do overall more damage in that time.
For example on 30 sec encounter it can suggest you the highest amount of spell damage and spell crit, when on longer encounter more mp5 and intellect.
Probably it do that, because calculated, that you should have enough mana to keep higher dps with Mantle of the Blackwing Cabal.