I just put my Tamron 60mm f2.0 macro lens on my D300s. In Aperture priority mood, the widest aperture reads as f2.2, not 2.0. Has anyone ever heard of this, or have an explanation as to why it might be happening? Is it simply a sign of a defective lens?
…and God saw that the light was good!
Try the lens on another body; if the reading is the same, there's a Tamron-Nikon mismatch in electronic aperture transmission. The difference between f/2.0 and f/2.2 is annoying, but inconsequential.
What you have noticed may be perfectly normal. Many (most? all?) macro lenses change focal length as they are focused and the change in focal length affects the aperture. Please read the discussion referenced below:
...and, if you want to explore further, here is another reference that leads to further macro-related discussions (in addition to the one I already referenced above):
There are no rules for good photographs, there are only good photographs. -Ansel Adams