Ok I might be wrong but it seams like the slot overrules the rating now, Anyone else notice?
I've noticed this only on pre-patch imbued items. And that's because some ratings changed during the last patch(es).
Yah there is still some items with an open rune slot for me that I cannot fit anything else on.
Yes.
That and also items with more rune slots / rating than possible on that certain tier. Reason: see above.
I know that it has been fixed for the (hopefully soon) coming patch. (That or rating will just be a number.)