To reply to the original question I see this frequently when I buy UA tickets on my Bank of America Visa (credit) card, and it seems to be especially common when there are two travelers on the PNR.

What happens is the original auth is taken for the full purchase amount but the auth isn't converted into a charge and posted. Instead the ticket is posted as a charge when issued (or for multiple pax like, a separate charge for each ticket is posted) and the auth (showing as a pending charge) is allowed to natrually expire (generally 72-96 hours out, though I have seen them linger longer)