For some reason you were charged the double amount of what you should have been charged. Both journeys should actually cost 16.78 euros since you have a 40% discount subscription and King's Dag is a national holiday, on which your discount is valid the entire day.
I'd advise you to contact customer service directly to have this rectified. Usually, this is most easily done by phone, as they are available 24/7 at 030-751 51 55. You may also try to contact them by chat (click the link Chat with NS on this page) or by sending a message on Twitter or Facebook.
€ 33,48 is the fare for a return trip from Vlissingen to Maastricht with 40% discount.
I'd advise to check the fare charged in Mijn NS at the website, as the app has some bugs in displaying trips lately. If the fare charged is wrong at the website as well, your best option is to contact customer service.