The worlds are constantly opening/closing depending on how many players are logged in.

The webpage you are looking is only updated 5 times a day (see right near the very top it'll say about times it's updated). Although the webpage is only updated at those times, the server continue to change.

If you're on a very populated server, the only way to get a character onto it is either:

A) Wait until a maintenance and do it just before maintenance starts (as people are logging off) or just after it ends (before people get a chance to all log back in)
B) Create a character on another world, wait 3 days and pay for a server transfer (which doesn't care if the world is open or closed)
C) Just keep trying, possibly in the early hours of the morning may work when people are asleep, but again this depends on how busy your server is and how many people AFK on it.