That's correct. I guess the only reason it's not called a F2.8-5.6 is that, by convention, the F number is defined as the geometrical ratio of the focal length (at infinity) divided by the effective diameter of the lens.
Years ago there was a move to re-define the light transmission value of a lens by its T (transmission) number, that would account for variations not only in its extension but also the actual attenuation of light passing through the glass. Unfortunately (I feel) it didn't catch on except amongst the optical technologists.
You are not the only one to be "caught out" by this. Only this month a respected UK magazine reviewed the Micro-Nikkor and actually implied that this lens "behaved oddly" because the camera reported (correctly) this effective change in F-number as the lens was extended. Far from odd, this is just what *should* happen and one could argue that all the others are "wrong". I've emailed the magazine concerned to suggest that its reviewer needs to revise his knowledge of simple optics before making such statements!
John Gruffydd Mold, Wales, UK D300, D200, OM1, OM2n, Bronica ETRSi, Lumix LX5