Because they are presenting the best deals to many customers as they search online, your booking is always reconfirmed with the airline reservation system right before they take any payment from you. This is why, once you have found the best deal, they encourage you to have your credit card and personal details for the traveller close by. If they cannot confirm your original booking request they always will offer you the next best deal based on your original selection. Unless you receive the Confirmation screen message and booking reference they have not taken any money from you.