Cex has 2 types of mining: the mining which you as an individual bring to the pool (like any other pool), and the hosted mining they offer for people to trade in BTC and NMC. The two do not mix. Cex themselves supply all of the hosting for the hashpower that gets traded on their market, i.e. Cex is in total control of the supply of hashpower in the market - they are supposed to act as "market makers".

The way that Cex does it is they add more hashing hardware in their hosting facilities and then they sell it onto the market at the market rate (which means that Cex earns the bitcoins for these trades) and the price is, of course, set by supply and demand. If Cex did not increase the supply of GHS in the market, you would actually see a rise in the cost per GHS because of more miners coming to the pool with greater amounts of bitcoin fighting over the same amount of supply (yes, this would happen despite the return on holding GHS decreasing as the difficulty increases).

So the reason why I left Cex, is because I don't trust them to add hashpower to the market efficiently in a way that has a minimal effect on the price. They could be as clumsy as adding 5TH/s and dumping it all onto the market at once causing the price to crash, or they could be smarter and dribble it in over time. You just don't know, but I guarantee they are not as smart or as experienced as the traders in a commodities market.