Many possible reasons. A1-15 are reserved for Business Select flyers. A-List Preferred and A-List members get the next assigned positions. This is followed by those who bought EarlyBird Check-In. Because Southwest places no limit on the number of EBCIs they will sell per flight, people often get B (and sometimes even C) positions, even after paying the extra fee. Then, after all of these people, are the positions available for those checking in at T-24. Another factor would be if your flight is the continuing leg of a connecting flight. Connecting passengers can check in for both legs (including the flight you're on) when they check in at T-24 for their first flight -- meaning they are checking in before you. If you need a specific seat or assistance boarding due to a disability, consider asking for a preboarding pass at the gate. Alternately, you can pay $30 or $40 at the gate to upgrade to an open A1-15 position, if available.
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