>Aren't most rings an actual thin circle outline

Aye, generally speaking, they are, but there are quite a few (around my way at least) that whilst almost invisible to most viewers, suddenly leap out in the right light, or in a rubbing. (Though RAuk does have a preternatural facility for being able to spot these subliminal carvings without enhancement).

There's a good example of this faint ring thing at Millstone Burn:
http://www.themodernantiquarian.com/post/44258
looks like a plain old cup in the daylight, but Stan the Man found allsorts going on in there when he did a rubbing:
http://rockart.ncl.ac.uk/panel_image_view_larger.asp?imageid=400&pi=430
the motif in question is the complex looking one second from the right.

Rockandy spent yonks scrutinising the complex bits of Millstone 6d, and more or less concluded that either

1. They had eroded completely
2. They were artifacts from the translation of the rubbings
3. They were on a completely different, but suspiciously similar rock.
4. There's no such thing as rock art, it's all a figment of the imagination

Sometimes, it can be the other way around where the rings are clear and tight, but the cup looks a bit flat and iffy. Such can be seem at Coldmartin Loughs:
http://www.themodernantiquarian.com/post/45543.

Either way, I reckon Goff has flagged up a serious bit of RA, it might not look like much, but RA up there is a rare thing. Combined with Fitz's example of Lewisian cups, I'm tempted to reconsider the idea that the slight depression on the surface of the central stone at Callanish may be the remains of a pre-existing cup mark. Quarried deliberately, in a Long Meg stylee.

*'Course you're not. More like the dispenser of a healthy tendency to question things. They're an ambiguous set of carvings, are Goff's cups, mebbe without you raising the issue, that faint ring would perhaps have remained below the threshold of general awareness :)