I suppose a silent auction has a hard event end but I dont believe live or verbal auctions end at a specific time. I thought they ended when bids stop coming.
Because it is the internet a longer grace period is given between non bids (3 minutes) to ensure no advantage is given to people with the fastest internet connection. Players should go to the highest bidder, not be some kind of game of chicken to see who can bid a half second before a hard finish. Not about the best and fastest mouse clicker, which is what it would denegrate to if a hard time finish was implemented.