What’s wrong with Yes and No?

Have you noticed that more and more software is using “OK” and “Cancel” in dialog box prompts when “Yes” and “No” seem more appropriate?  The software installation I ran today presented me with a dialog box similar to this one when it was finished: 

The answer to that question should be “Yes” or “No”.  I don’t know who decided that “Ok” and “Cancel” would be easier for users to understand.  My understanding is that Apple strongly discourages Yes/No selections.  If somebody can give me a reasonable explanation as to why and how Ok/Cancel is somehow better, I’d be grateful.