The amount does not matter, it is still considered false accounting/fraud.
You said you can only refund it if they complain, please send a newsletter to ALL of your customers asking them to check their account and ask for a refund if there are discrepencies.
No scam involved at all. I will try to make a several-page-long technical report short and explain what and why is happening right here:
1) Let's say 'username1' has been doing some cloud-mining on CEX for a while. He owned a certain amount of GHS and it was all fine, until the cloud mining was suspended.
2) His GHS produced a certain amount of shares all the time. Miners, however, accept those shares in batches. 64, 128, 256, 512, 1024, etc. depending on the size of the miner. Smaller miners accept smaller batches, of course.
3) When cloud mining was on - it was not an issue. Providers had all kinds of equipment of different sizes constantly on and all the shares got accounted for.
4) Now that cloud mining is off, this is not the situation. Only so many machines are on that are needed to support the users who have turned on their GHS. Mostly those are big machines, because they are more efficient. Those machines can accept, for example, batches of no less than 1024 shares.
5) 'username1' has submitted 756 shares before mining was switched off - technically he has still not paid for them and owes the mainenance to cex, but the miner has not accepted those shares because the amount is smaller than the minimum batch size.
6) 'username2' decides to turn on his cloud mining. He has a small amount of GHS, and it so happens that the equipment provider needs to turn on a small miner to cover for his GHS. This small miner accepts 128 shares as smallest batch. Recalculation happens at the moment the miner is turned on, and out of 756 shares that 'username1' owes CEX 128*5=640 shares are accepted. 756-640=116 shares remain unpaid for because this amount is smaller than 128 batch.
7) This situation can repeat over and over again, as smaller miners that accept 64 or fewer shares are connected and disconnected.
This works both ways - if CEX owes user rewards for unaccepted shares - those are paid out in the same way.
9) From user side this is viewed as a glitch - he has 0 GHS and/or his mining is off - he does not have to pay maintenance. From CEX side this is no glitch - this is math.
10) This situation is always unexpected, as miners turn on and off when users press the button - we do not control the process.
Taking into account all of the above - CEX.IO views this situation as such that does not require a refund to the users affected. However, as courtesy to our users and to avoid negative publicity and unsubstantiated accusations we take those expenses and refund the affected users on request.