Originally Posted by
Often1
There are thousands of people online looking to book at any moment. You didn't book "immediately". It may have taken a fraction of a second, but that is not immediate. Someone else got the cheaper seat.
The only "but" is that the lower rate is still offered 24 hours later on their website, until you want to reserve it, when it goes up again by 50 %...

Maybe they are working with fractions of a month in Belgium...