Tesla always errs on the side of unavailable when they do not definitively know that the port is available. When a SC goes offline, and cannot report its status (for whatever reason), they report it as busy.
Keep in mind it says (at least xx of yy ports available).
I have has this happen in Milford, CT at the charger at the mall. It reported that at least 2 of 14 were available. When I arrived, there was no one there. The funny thing is that I called Tesla support and the woman on the phone told me about this method. I mentioned that I wanted to make sure that I had not missed the invisible Tesla upgrade that the other 12 people got. She responded without a beat and told me that I should get that update in a week or so.