Again not refuting you instace of this happening just asking for others to validate here as well. A sample size of 2 refuting occurances which we have here is not large enough to validate either possibility as the defacto.

As you say and I also know creation is available live based on currently logged on accounts, refreshed server side more often than a client is permitted to check availability. A) nothing is stopping the availability opening on happenstance for the brief window, however statisticly unlikely from your attemp spam. B) rmt bans would be a valid gauge of account being actively logged in, as they do not generate revenue while offline and therefore have almost 100% uptime logged into their respective servers.