I think the problem is that people try and put it all into a one dimensional line of non-monogamy like:
Swinging -> Open -> Poly
(sexual non-monogamy ---> loving non-monogamy)
whereas really, because of the huge differences in people, it's not a line but some sort of complex 3d shape, and each relationship will include different bits of each.
To me, if I'm in a committed relationship and I have a FB on the side, and neither of the two partners knows each other (although they do know OF each other) then that's still poly. However, some will say that since they don't know each other it's not poly. Others will say that since I only have one committed relationship, it's not poly. Someone else might ask if I would be open to a second committed relationship, and if that would make it poly.
The problem is that, often, one word is not enough to describe the complicated dynamics of poly arrangements, and even if it was, some people aren't fans of the labelling aspect anyway. I find that it's more helpful to worry about the actual problem rather than what box you want to put it in.
I think there's some people who just want one major relationship, and everything else fairly casual. To me, that's still poly, but obviously people will mainly post here with their PROBLEMS rather than the things that are going right. If I want one relationship + casual sex on the side, and I've got a great relationship, then I'm not going to ask about it.