I certainly got that a zoom lens may have a different max aperture at the short and long ends.The maximum aperture for a zoom lens may be same for all focal lengths, but it is more common that the maximum aperture is greater at the wide-angle end than at the telephoto end of the zoom range. For example, a 100 mm to 400 mm lens may have a maximum aperture of 4.0 at the 100 mm end but only 5.6 at the 400 mm end of the zoom range.
Let's look at another resource, say the manufacturer of your 4/3rds cameras of choice(Olympus):
Olympus/Zuiko makes some variable max-aperture zoom lenses for their 4/3rds cameras. I especially like how they have DOF values on their website for their lenses.
What I can't understand is why there is no DOF info on these lenses at the middle range or long end for say f/2.8 and no DOF info on these lenses at the long end for say f/3.2. Could it possibly be because Olympus/Zuiko does not know how to "lock" the aperture at f/2.8 in Av mode for the entire zoom range, or because zooming these lenses causes the max aperture to change from f/2.8 to f/3.5 as it says in their literature/website. Hmmmmmmmmm?
edit: I hope that this is not just a limitation of the 4/3rds format.