Confusion over Billing

Whenever I have taken new contract lines, I was always informed that we are billed in advance. From what I understand this means that the first month is pro rata and at the end of the month you are billed the pro rata and the next months subs. The contract in question was migrated to pay as you go at the end of March as the contract term concluded mid March. I was however still billed at the end of March. Is this not contrary to billing in advance.