On the ES dome right now we see size roughly 200+ to 600+ contracts at a strike. If someone wants to sell 499 at 1373.00 and market is fluttering there, trying to cheap out the last tick may cost them huge $$ on a missed entry or exit.
Open interest on the ES is not unlimited as some people think. When block traders need to fill part of spreads, hedges or outright positions, one tick does not keep them from placing the trade.