The billing period for any account is always from the 1st until the last day of the month, and invoices are created on the 1st of each month. But what happens if you subscribe in the middle of the month? In that case, your first month is always prorated.
When you subscribe to VIF Internet, we ask that you send your first payment by mail or by make the payment by phone. The payment of course covers one month billing, so a credit is applied to your following invoice. That is the reason why that invoice is less than you expected on your first month.