On the first page where you choose flights you will see this disclosure at the bottom of the page:
The prices shown are intended to help you choose your most appropriate journey. The exact total will be shown after you have selected your outbound flight and may be higher than the amount shown on this page.
My understanding of the reason for this is that the site (similiar to lots of travel agents/booking sites) at this point only checks availability at certain intervals.
Only when you continue to the next page where the total is shown is actual live availability checked hence the disclosure.