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