They started their example pattern with an citation number 984,946,606 they earlier said wasn't valid rather than 984,946,605 given initially (and shown in the image).
Because it means that 85.7% of all mistakes will be caught by very simple software checks before getting to your system. (85.7% == 6/7).
Check digits in your userdata is an old trick and is very useful in practice. Maybe modern systems should aim for something better than %7 but it's a good starting point as a system design concept.
Just a mistake, I imagine. Probably just typed the pattern out starting with the last number they wrote, which unfortunately was invalid.
> I was looking at ticket 984,946,605. When I type in 1 higher, 984,946,606, no ticket is found. ... So the ticket after 984,946,606 is actually 984,946,610
They started their example pattern with an citation number 984,946,606 they earlier said wasn't valid rather than 984,946,605 given initially (and shown in the image).