Have you travelled by bus, tram or metro? The NS Flex subscriptions are only valid for trains. For other modes of transport you will always be charged the full fare. It could also be that some correction fees are being charged.

In any case, you can find a detailed specification of the costs in Mijn NS (go to Facturen). That'll probably clear up the confusion.

You could also contact customer service directly; they'll be able to tell you what you are being charged for. Usually, this is most easily done by phone, as they are available 24/7 at 030-751 51 55. You may also contact them by chat (click the link Chat with NS on this page) or by sending a message on Twitter or Facebook. Customer service can also help you if you have been charged too much.

OR you did not do either checkIN or checkOUT correctly-forgotten or not on same transportcompany as IN. In every case this will lead to a charge of 20 eur extra. But as said: it should be obvious to first check all the registrations. Same as by a huge fone bill or whatever.