They typically make more offers than roles they have available, as they have fairly good analytics around what % of offers will not be filled post acceptance (ie someone simply never shows up for day 1 of work).
Now, fast forward to today, and as you've heard in the press, and from Amazon directly, they are slowing hiring and may even do cuts in the new year. I think what you're seeing is in some areas they may be over running offers, and need to pull back and kill some post acceptance to correct based on the new reality of hiring forecasts.
Still sucks and is a shitty situation.