Norton Safe Web Question

Hi there, I have question about Norton Safe Web.  I have noticed that some websites that I put into the Norton search do not come up with results.  However, when I put them into google, they come up.  When they do come up in google, they have the green norton "OK" square next to them.  Now I understand that nothing is 100% these days but does that mean those sites have been tested and deemed safe and I don't have to worry about drive-bys and such?  how does Norton accoplish this and are they pretty accurate/up-to-date?

 

I am rather wary right now because my NIS 2012 blocked a drive-by a couple of days ago when I had hotmail, msn messanger and this very forum site open so just want to get some clarification/reassurance....

 

Thanks