The primary reason for the difference is as a result of your services coming online during the previous bill period. Normally, rentals and features are billed in advance, one-off events and usage are billed in arrears.
But as your services came online during a bill cycle, it was not possible to bill for last cycle's rentals and features in advance and they need to be recovered in arrears.
Therefore you have been billed a prorated portion of the period that has just ended in arrears as well as the current bill period in advance.
In addition to the inclusion of prorated charges, you may have been billed for any once-off charges that were agreed at point-of-sale, such as Router Postage, New Line, Restart or Renumber charges, together with any applicable Engineer fees.
Your first bill will cover the cost of your plan and any additional services, such as:
Your monthly plan charge - charged one month in advance from the activation date
Add-ons – such as international minutes or call packages
Device charges – if you pay an amount towards your device, or a delivery charge