Yes Origin held those viewers for longer. Part of that is because it is event viewing, audiences don't want to turn off the channel in case they miss something. Same with viewers flicking onto coverage, they'll watch for longer than a standard NRL viewer. Hence in an Origin you convert more 1 minute standard reach viewers into 15 minute viewers etc.
The thing about reach is -- if an NRL game went for as long as an AFL game - say 160 minutes+ etc - the reach would actually be similar. There'd be the same amount of casual viewers flicking over during the actual game time. We'll all see this come summer when we have the reach for nationally broadcast cricket matches in 3.5-4 hour prime time blocks. Although cricket now has lower appeal than both footy codes, you're simply just casting a broader net/time to pull in people who are flicking/swimming past.
I'd argue the main advantage that NRL has over AFL - and this comes back into those reach vs average ratios - is that there are fewer breaks in play. Whereas traditionally a break in player has been viewed as great for plugging in a commercial break, it's now widely seen as an opportunity for viewers - who are less committed - maybe even more ADHD - to change channels. Every quarter and half time break, every ad after a goal, you're giving an opportunity to an AFL viewer to ask themselves: "Why am I still watching this when there's 10,000 other things I could be doing?"