seem to recall, several years ago, an announcement by some exchange that they were implementing "shadow" book orders, that are invisible to most ordinary clients. Perhaps they just started showing shadow-book trades on the ordinary ticker, to boost their public volume numbers...
That seems to be what's happening. But why would they want a lot of off book transactions? Don't they make money based on a percentage of the order cost? Why would they not want the price to rise as much as possible?
OKCoin and Huobi do not charge fees for trading, only for deposit and/or withdrawal, and interest on leveraged trading. As I recall, the shadow book was meant to cater for clients who wanted to trade on the exchange, but did not want their large orders to affect the price against them. (I may have misunderstood, and I don't know whether that makes sense.)