I think that could lead to open arrangements where those involved don't want to know about outside liaisons. I'd say that sort of thing would be the way to handle folks who want the thrill of the hidden.
I also had a thought that maybe she's hooked on NRE and simply bounces from fling to fling to get it. The makeup period between flings could also provide some of that feeling, too. The love of the NRE, however, doesn't explain the continued pretense of monogamy and cheating, though I suppose that could be driven by guilt.
When speaking of various forms of non-monogamy...it ain't poly if you're just fucking around.
While polyamory, open relationships, and swinging are all distinctly different approaches to non-monogamy, they are not mutually exlusive. Folks can, and some do, engage in more than one of them at a time--and it's all good.