The product price was a different one, but it changed when I went to checkout page. How can it be?

On yithemes.com there are some time-based offers that run randomly every day. They are also limited to a given number of licenses, meaning that if some other user has bought the last license while you were on cart page, the offer is expired and your plugin is back to the regular price. Don’t worry about that, though, there are many offers running every day on our plugins, you will find the one that suits you!