Yep, that is exactly it.
If you have an iPhone you haven’t rebooted in a year, and you have an app you ran a year ago and never did a force quit on & never ran again, it is still in the list of apps. It is highly unlikely to still be in memory, but it is still in the list (it might still be in memory if you have managed to have that phone for a year and never run very many apps and none were memory hungry…or say you got a newer phone and this one sat on a desk attached to a charger for a year unused).
I think that is actually the goal. Whomever designed it decided people want a list of apps that they had been running, and it is an implementation detail if that app will need to relaunch or not, so don’t bother people with it.
It isn’t a bad choice, and back when the first iPhones came out and RAM was extremely tight it was arguably a great choice because that list would have only had one to three apps in it & people generally want a computer to remember more then that for them.
I would personally like the “in memory” apps to be distinguishable. Badges for ones that actually run in the background (like a little phone for VOIP apps, a note for music/podcast/audio apps, a stopwatch for ‘about to be put to sleep any second now’, and merely bold text for ‘still in memory because we haven’t killed it, but it isn’t even running’), however I’m not head of design at Apple. I mean I’m no longer at Apple, and wasn’t in design at all when I was, and this sort of UX is really the kind of thing that only the design team is going to get to do, and only people really high up on that list…
The current head of UX prefers things to be as invisible as possible. Like on th eMac he prefers common controls be invisible unless the mouse is over them (like the remaining time on the current song in iTunes, or if I’m wrong about that exact thing, similar stuff, like on a huge monitor you need to be hiding everything all the time?).
Anyway, that is a whole lot of words to say: yeah, it wasn’t written down anywhere, and your mental model of what that list means while wrong was utterly reasonable. You ain’t dumb.
Of corse nothing really gets written down for computers anymore - I can hardly blame Apple (MS, or anyone else!). Manuals are expensive to write, and basically nobody reads them. It is still a shame though, maybe less then 1% of people would read a manual, but I bet the few that did would get a lot out of it. Plus we would have more variation on the endless stream of articles about “iPhone secret tips” and “Secrets of the Windows masters” if the people who wrote that sort of stuff had any real access to information 
