Explained in article.
Roger, after tearing down both OS versions of the 120-300, said it appeared that elements in these lenses could be interchanged. The optical design seems unchanged. When questioned about this, Sigma replied that the 120-300 "S" has 2 FLD Glass elements instead of one. FLD glass has performance equal to fluorite, which is excellent.
So 2 FLD elements instead of one so better IQ.
One major problem with that... if you look back to the older one, Sigma says that has two FLD elements, and further references to this can be found in various reviews like at photozone and lenstip. The only way this unspecified source at Sigma could be right now is if Sigma were originally lying about having two SLD elements. Doesn't add up.
I'd speculate it could still be down to sample variation, especially if Sigma have tightened up their build quality.