sorry about that , it wasnt quite clear to me from your original post that you understood how an availability display works . ( and it is something which , in my experience , is often misunderstood )

with regard to your original question I can only say that even airlines themselves can be puzzled by how loadings change - when travelling subload I would sometimes get on flights that 30 minutes earlier I had been told ' not a hope' and conversely I would be bumped from flights that I had been told to expect 'wont be a problem' . In some cases ( and I am not saying that this is the case with your flight ) all it needs is a large family group travelling for an unexpected bereavement to change the picture completely and in other cases a late mechanical problem with a competitors flight on the same route can make a last minute difference between an average load and a full one .