Originally Posted by stevebrot 
The behavior varies from lens to lens. Very strange.
Steve
That is quite possible, but it could also partly be a different point on the same curve. Going by the progression, a lens that underexposes at f/1.4, but exposes correctly at f/2.8, will probably overexpose at f/8, if you see what I mean. The curves may be in different places for different lenses (that is, the point of correct exposure may vary), but the general trends seem to be similar.