Well, Naxx can have two ilvl increases:
In MC are 9 Bosses (Excluding Ragna)
In BWL are 9 Bosses (including Ragna)
In AQ40 are 9 Bosses (including 3 optional)
IIRC Naxx will have 18 Bosses.
And it is the only 40 man raid instance until next year, so it could be that Blizz counts it as two 40 man instances in terms of ilvl increase. (But I don't think that's likely.)
In MC are 9 Bosses (Excluding Ragna)
In BWL are 9 Bosses (including Ragna)
In AQ40 are 9 Bosses (including 3 optional)
IIRC Naxx will have 18 Bosses.
And it is the only 40 man raid instance until next year, so it could be that Blizz counts it as two 40 man instances in terms of ilvl increase. (But I don't think that's likely.)