This is perfectly normal.
We refresh our list of deals on a daily basis, so there could be a change every 24 hours.
There are a couple of reasons why deals change that we cover in this article but the crux of it is that retailers are competing for your business on price.
What we suggest is that you get on a better deal when you first use the service. Then, set a price alert (more about that here) to help monitor the market for you every day.
If anything better pops up, or if we know that your deal has been changed, we'll let you know and you can take action!