The molecular weight is only approximate in SDS-PAGE analysis.  Two identical mass proteins may have different migrations based upon their Stokes radius and charge to mass ratio.  The radius is affected by how distended the molecule is (reduced versus non-reduced conditions for example) and the charge/mass ratio can be affected by the amino acid composition, size of the protein, sample treatment, and post-translational modifications.  Probably your molecular weight standards were already heat treated but not reduced?

The molecular weight is only approximate in SDS-PAGE analysis.  Two identical mass proteins may have different migrations based upon their Stokes radius and charge to mass ratio.  The radius is affected by how distended the molecule is (reduced versus non-reduced conditions for example) and the charge/mass ratio can be affected by the amino acid composition, size of the protein, sample treatment, and post-translational modifications.  Probably your molecular weight standards were already heat treated but not reduced?

A protein may run anomalously on SDS-PAGE if it has a disulfide bond but isn't reduced, if it has a very stable secondary structure but isn't boiled, or if it has a post-translational modification. Did you check its mass by mass spectrometry to see if it has a post-translational modification?

If you look at a text book log-linear plot of protein mass vs. mobility for a variety of proteins you will see that they are scattered around a line, not right on the line. In other words, mobility is not a perfect predictor of molecular mass, for reasons that Grant explained. The store bought MW markers are proteins that fall on the line or have been engineered to fall on the line. Reducing agents are pretty important for running a Laemmli gel, but if reduction is critical and you leave it out you usually get a smear or a ladder of multimers in my experience.

I can't resist playing with the interpretation of your question. Are you asking: Why does my protein that I've never seen on a gel before, but has a predicted MW of 9 KDa, run at 11 KDa (relative to store bought MW markers)? Or is the question: Why does my protein, which I usually observe to run at 9 KDa, now run at 11 kDa? For the first question, I think the answer is probably mine and Grant's. For the second case, Adam may have the answer.