Can someone please help me understand what's going on in this order book? This is a screenshot of the XRP/USDT order book from Poloniex today. I have often noticed that there is a sell order and a buy order of almost the same size (usually quite large) and I think it is the same person doing it (either manually or using a bot)

Most likely this is a robot that earns small amounts on the difference when buying and selling. Also, market makers often place large orders and move the price in the direction they need. When the price approaches these values, their order is shifted by several points.