Take a close look at the vertical alignment of the front sight--I'll bet it's canted to the right due an improperly indexed barrel installation.
My 10.5" SBH, which was purchased new in '03, had the same problem. I had to crank the rear sight blade all the way to the right to compensate. A quick call to Ruger's customer service resulted in a call tag being issued. Ruger returned the gun, barrel now properly indexed and front sight correctly aligned, in about six weeks.