Considering my guild is still working on TFB 16-man HM you can take my stab in the dark with a pinch of salt.
From watching every TFB HM vid on youtube, i'm gonna take a guess at the bug that these guys are talking about.
P2 boss starts at 50%. 6 Tentacles in P2, 5% each. But if the boss enrages before the last tentacle is killed the boss gains 200% damage multiplyer buff which unintentialy buffs the boss hp drop from the tentacle kill. From watching the MOX kill you can clearly see the boss lose 5% when the tentacle is killed at 8.22 but when the boss is enraged at 14.40 you can see the boss lose 10%.
(You're welcome for the free plug! and i enjoyed the vid thoroughly)
The strat used was clearly very smart as to reduce the hp in the burn phase to a managable amount, it's just a shame bioware once again failed to deliver polished content.