I think it's something to do with an empty planet production queue. Occasionally I'll see an empty planet start producing Aphrodesiacs, for instance, even though I'd already built them a long time ago in game time.
I *suspect* that what is happening is that the Governors are ignoring the options available to a planet and instead are choosing from a list of things that you have the tech to build, but have not built on that planet yet. Usually, that's not a problem, but sometimes the Governor decides that a trade good is what you really need, or you should try getting a certain wonder, and it picks that.
~SDC~