I'm aware of overselling, but in overselling, well-established airlines presumably want to avoid an overly aggressive sales policy and deny boarding to ticketed passengers too often.
Earlier this afternoon, I changed a (already checked in) flight for tomorrow morning - as soon as my change went through I could see a seat magically appear on the airline website on what was previously a sold out flight so the computer has clearly decided it can sell one more seat. The question is then why the airline didn't allow customers who were already looking to register interest in the same flight so they could have (re-)sold my seat more quickly