Why does the reference code change when I reopen the SEPA payment page?

Each time you select the "SEPA Bank Transfer" and click on "Continue to Payment", you will be shown a new code which you can enter as a payment reference for the payment in your online banking, or on your transfer form. 

Each code is valid for 7 days. If you open the payment page for the deposit e.g. 10 times and 10 codes are generated, you can use every code. The only important thing is that when you make the final payment, you go back through the payment history and generate a new code.