yeah, for "[HealthFraction]", this could well be the case. i could see how for a boss with a huge amount of health, reaching "0%" could happen well before the boss is anywhere actually on the point of death (could have quite a few HPs left).
Aloft actually doesn't use UnitHealth() / UnitHealthMax(). one reason is that Aloft doesn't always have a unitid to use to drive those functions (i.e. in a multi-target/AOE situation), and another reason is that (now, in 3.0.2+) the the Blizzard StatusBar object that comprises the health bar in the underlying/default Blizzard nameplate "correct" health values in it, and Aloft just uses those (posted via the "OnValueChanged" event handler and gathered via StatusBar:GetMinMaxValues(), which is all a lot easier and more efficient that a health estimation addon).
but Aloft does calculate fractional health itself, from these values... so i will go make certain that ceil() is used properly in that calculation, just as you suggest.