Offhand, I don't recall any specific likelihood being assigned to different encounters. I believe it's just the base chance of an encounter followed by picking one at random. However, they do have a tendency to reference the same enemy party multiple times in the array of available encounters. All encounters are definitely not equally likely, but, except in cases where they put one party in 10 times to every other party's 1, it'll take an awful lot of testing in every area to sort out the probabilities.
In a theoretical zone where the list of encounters all point to unique parties, all chances should be even, though.
Edit: I could have sworn I already investigated "Let Go" in the code. Turns out it was over at GameFAQs.http://www.gamefaqs.com/boards/198844-s ... /807580276