Your invoicing date is a fixed date and not related to your active subscription at all. Considering one month of WeekendVrij is about twice as expensive as what's on your invoice, these are just the costs of your subscription for the ~ 2 weeks between your previous invoice and the day you cancelled the WeekendVrij subscription.

Thanks, so does that mean I will get charged for 2 months, even if I cancelled on the first day of the second month? The reason why it’s cheaper for me is because I got the weekendVrij on 50% discount (when I signed up), so it should only be EUR16.91 for 1 month, but I got charged that for 2 months, instead of 1, even though I cancelled on the 1st available date on the second month. 

You'll get a refund for the unused days from Weekend Vrij, because the change is effective immediately. With this change your new season ticket will also start and you'll be charged until the next prolonging date, which is sometimes not the whole month. 

Let's say your invoice date is every 1st of the month, and you change your season ticket on the 20th, then you'll get a refund for your old season ticket from the 20th to the 1st and get an invoice for the new season ticket from the 20th to the first, after which a new month will start on the first.

Only when changing a subscription on the very first possible date, the next month will not be invoiced.

When changing subscriptions somewhere in a running month, you will be invoiced for the next month but will get reimbursed for the unused part after changing/cancelling it.