Sig 18-50 f2.8 EX DC Macro & Sig 17-50 f2.8 EX DC

Apologies first - I'm very much an amateur without a lot of knowledge.

I bought the 18-50 EX DC for Studio backdrop shots of work (I'm a Potter) and am really pleased with the quality. (I did know enough, from reading her, to check the lens as soon as I rec'd it and am lucky to have a very good copy). Also love it for shots of wild plants and the really definable OOF.

My question is: why is the Sig 17-50 EX DC so much more expensive than the 18-50? What is it about the glass, elements etc that make it superior?

Thanks for any and all replies,

Kevin