It almost sounds like the emergency lighting circuit and the "dirty" or regular lighting circuit are both running through a common switch box while the emergency does not terminate at that point and just feeds on through, whereas the standard lighting circuit switches a portion of the tubes/bulbs at the same fixture. The emergency circuit typically is tied directly into the battery pack of the fixture normally from a completely different panel, unless its 1 circuit that provides a constant hot for the battery and a switched version for the general fixture lamps. That may be legal, I'd have to look it up, using one circuit to be manipulated for a constant and switched design of a combo two purposed fixture.