It is because the bot counts the time not the orders.

​

So for example

​

In the first hour there were say 100 orders. In the second hour there were 50 orders. The third hour 25 orders and so on ad infinitum. Imagine you made an order 10 hours after the start. The bot doesn't understand that there were only 1 orders made at the 10th hour. The bot only counts the time. So while watching the bot your "time slot" will go from 0% to 100% way faster than somebody who was in the first hour as the first hour ended 2 weeks ago in terms of orders.