difference in bill and actual costs

You can find a detailed invoice in Mijn NS (the website, not the app; go to Betaaloverzicht/My payments) to see what you are being charged for. On the first page, you see the total amounts, but if you look at the later pages of the pdf file you can see exactly for what journeys you have to pay what amount.

If after looking at the invoice you still have a question about it, or if you can't find the detailed invoice, then you should contact customer service directly. Usually, contacting customer service 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 X (formerly Twitter) or Facebook.