That's because it's an estimated time based on average queues of people recently queueing, so it's just giving you the most likely time frame. At the same time it's still trying to match you up as quickly as possible so if a spot for you suddenly opens up sooner than the estimate (say because people left or a whole bunch of the most needed role happened to queue at once causing it to go threw the waiting list much faster) you get the situation described.
