As rileysan mentioned countries like Russia and alot of the arab states did massive amounts of CTO's for collecting or tourist purposes. These countries basically flooded the market although they make for good thematic collections. This is just a guess and may be backed up or not but if the China CTO's are doing that well Cancelling to order may have been a less used process and thus making them more rare than the same typically used or even mnh stamps. I'm not sure of quantities for various China stamps but with the high population I would imagine Regular defins and commems were plentiful making the CTO's actually a rarer stamp amongst its postally used or availiable same stamp. More and more the trend is for used to be more valuable since many new releases are done in low numbers and a good portion are being collected MNH . For example the new Canadian $10 blue whale that can still be purchased thru
Canada Post has sold used for $20+ . China is a very unique and misunderstood country from alot of aspects but generally they do things for a reason and usually its to make money.