On a donation, or a form with payment, all the payment fields get validated immediately to check if they have the right format (e.g. if the email field really got an email address, or if IBAN, credit card,.. have the right format and number of digits). Then, Campaignion sends the data directly to the payment processor, e.g. either Stripe or Paypal, depending on the selected payment method.
Now, most payment providers don't give any feedback if the payment worked to the site, if it worked or not, it just redirects back (or displays their own "failed donation" page).
Also, for the payment method called 'manual direct debit', Campaignion cannot look into the account information, and it would also take to long for Campaignion to wait for the answer of the check. So it just saves the supporter data and carries on.