There could be a number of reasons why you have to pay more than you expected. For example, if something goes wrong checking in or out, NS will charge you a correction fee. And also note that NS Flex subscriptions are only valid for travel by train. If you travel by bus, tram or metro, you will always be charged the full fare.

You can find detailed invoices in Mijn NS (the website, not the app; go to Betaaloverzicht/My payments). There you can see exactly why you were charged 203 euros the past month.

If you feel that you are being charged too much, or if you can't find the detailed invoices, then you should contact customer service directly. They have access to your travel history. 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. Note that it can take a few days to receive a reply if you contact them online.