The issue is that you select a time to start the booking process but that time can change while you go through the process, especially at 7AM when the system is being slammed.
Because it's been a few months now I forget the details of all the screens, but the first time I tried to book ROTR in November I was trying to get a time around 5 PM. So when I clicked on the screen to book they present you with an hour to select, presumably starting with the earliest hour available. So, if you then click on 4 PM, it shows you specific windows after that hour, but they all might be after 5 PM. so, then you select a time like 5:10, and you have to accept that, and it takes you to a screen to select a payment method. I have my credit card saved to the app so I click on that.
On occasion, the system makes you go through an authentication step and sends you an email with a code to enter. If that happens you enter the code and are presented with a screen to confirm the purchase. By the time you get to there the time you are being asked to confirm may be still later, and when you press confirm you get a confirmation that isn't necessarily the same time as on the previous screen.
This whole process might not take more than a minute, but when I did it that time I started clicking on 4 PM and by the time I was done the time I received was like 6:40.
Some people have reported the time changing even after they received the confirmation. That has never happened to me; there is speculation that it's a result of the ride going down and pushing return times out to spread out the crowd.
My hope would be that with less traffic at 7:11 or 7:17 these changes would be less dramatic.
There are threads on this subject that you can find by scrolling through the main page of this board.