From my experience I think the odds of spawning a good instance change over time depending on how much they are farmed. It would make sense from a background simulation as if there is a known hotpoint where pirates get slaughtered I would imagine they would soon stop visiting.
During the zaonce CG I could maybe get a decent spawn at the local res 1 in 10 times and it seemed to drop as the CG went on. Flew to the main alliance space and almost every time a decent RES spawned over the next couple of days.
Of course that is only anecdotal evidence, I could have just been really unlucky, no way to tell really but I've been to enough to conclude that not all RESs are created equally, whether or not they change over time I'm less certain of.