I would guess you made another trip before this one, for which you checked out less than 35 minutes before checking in for this one. In that case, NS sees both trips as one journey, and also applies the same fare to both trips, i.e. the full fare for the second trip since the full fare also applied to the first. You can read more about that here, especially the part after “The first check-in determines the rate. How does that work?“

In the future, you can prevent paying for the second trip (in the off-peak hours) by using a different ticket for the part during peak hours. An eticket is usually the most convenient.

If you didn't make another trip, you may have checked in too early. The check-in time determines the rate for the whole trip, not the departure time. You can check-in from 18.25h forwards, then you shouldn't be charged for the trip.