What is wrong with my exposure meter?
I had the same problem with my Sigma 10-20 on my K-5 except it was when using the viewfinder rather than live view. I never tested the issue with live view. Pics were seriously underexposed and I had to increase the exposure compensation by as much as +2.0. The problem didn't seem to be with the lens directly because the lens worked perfectly with my K2000 and K-r.
I finally sold the lens and replaced it with a Tamron 10-24.
Allan in Colorado, USA