New Auction API: auction IDs staying, quantities changing?

I’m seeing commodity auctions with the same auction ID change between subsequent hourly data updates. Sometimes the quantities go up, sometimes they go down. I thought an auction, once posted, remains until bought out or expired? With the any-amount purchases of commodities I assumed it’d be possible for just parts of an auction to be bought out, sure, but how are auctions INCREASING in quantity without somehow resetting the time_left field?

Did you find any information regarding this?