Explain 'stat weights' to me please

I want to use wowreforge, the website for gear optimization just to check it out. I set the desired exp and hit stat, but it tells me it's not possible, even though it's more than possible for me and I've done it every time I get a new armor piece. This must have something to do with stat weights. Can someone please explain the term to me so I can use this site as it's supposed to be used?

Thanks in advance!