Wow, I'm so happy you brought in these findings because they were on my mind from the moment I started reading. I got my M.A. (having done my M.A. thesis on relationship abuse) at the university where Johnson taught before his retirement, but my understanding of situational couple violence is actually a little bit different; here it says that it refers to people who "regularly" resort to violence, but I think I remember reading that it's often more of a "situational" response than a routine thing.
For example: there's an argument, it escalates rapidly, and someone slaps the other — completely out of "character," given the relationship's previous lack of physical conflict. Granted, "situational couple violence" might also be a self-defense measure: a woman is being intimately terrorized without fighting back and then one day gets so scared that she grabs a knife and uses it on her partner, maybe even killing him, before he's ever actually physically assaulted her. That can be "situational couple violence" as well: that somebody, one day, just snaps. Men sometimes like to cite these statistics because they think it's evidence that women are more dangerous to men than vice versa... but they're wrong — because women (at least in hetero relationships) are exceedingly less likely to perpetrate intimate terrorism, which is (statistically speaking) the bigger societal scourge and does LOTS of damage.
Intimate terrorism, meanwhile (but not always), tends to involve a grooming period of non-physical abuse prior to the physical violence. In other words, an abusive dynamic is gradually built up, and what is clear from the research is that it gets worse and worse with time: more intense, and more forms of abuse come into play (say, first it's "just" emotional abuse, then it becomes financial too, and all of this gets more frequent with time... but maybe it never gets physically violent until a couple of years in — as an example). Either way, through allllllll of the abuse — even before it gets physical — there are deleterious physical and mental health consequences for the victim.
Okay, sorry, looks like I've totally nerded out here, but thank you thank you thank you for this article!