It seems like you already had NS Flex and then changed to a different subscription. In that case, the day of the month the invoice will be sent stays the same. So after a change of subscription you first get an invoice for the remainder of the current subscription month, which I suspect is what you have to pay the 4.03 euros for, and then you simply get new invoices every month with the subscription costs for the next month, which I suspect the 38.35 euros is for.
You should ignore the widget that says 76.70 euros subscription costs in Mijn NS. It's broken and often shows incorrect information, especially after a change of subscription. You will not be charged this amount.
Note that if you go to Mijn NS (the website, not the app), you can go to Betaaloverzicht/My payments to find detailed invoices. There you can see exactly what you are being charged for, and if it's for a subscription, for what period you're being charged.