I suspect it will be the difference with the old bill. If you change anything the first bill after the change is almost always different to cover the difference in days. i.e if you changed 5 days after your normal billing date 5 days of the next bill will reflect the previous billing costs.
Welcome into the Community, it's great to see you and also that you've already had some helpful advice.
This could be the case and I would like to discuss this with you but I would need to verify the account first. If you can have a little look inside your Inbox, you'll see I've sent you a Private Message. You can find this at the top right of the page, purple envelope.