In the vast majority of cases we've seen, starports outweigh outposts, which outweigh planetary installations. Beyond that things are less clear. There were some dev comments that market value plays a part, presumably if they're the same sort of place.
Now the caveat: We've witnessed several exceptions. In at least one case an outpost was taken before a starport - and the outpost doesn't even have a market, so as with your example, that isn't it. That's more extreme than other examples, where for instance a starport with a busier and more valuable market in every way imaginable was not chosen over another. We bat theories around about hidden values, like the old tiny/small/large/very large/huge asset markers and what they might represent, but...well they're hidden.
As always there's two reasonable broad assumptions: 1) the rule is complex and we don't know all the factors clearly enough, or 2) the rule is simple but noticeably-often doesn't work as expected/intended.
/shrug.
Edit: I'd report what happened in your case as a bug I guess, and see what they say. Even "this is working as intended" will teach us something, albeit frustratingly unhelpful.
That doesn't sound right to me. Would be worth dropping a report on the bug forum.