Our users were noting that the order number on the agile(scrum) backlog is skipping numbers (see attached). After some investigating, it appears the missing order numbers are tied to tasks on projects with a On Hold status. I did some testing in our Sandbox and this behavior appears to be the same with a Dead project status.
In the cases where projects are put on hold, I would assume the stories are taken out of the backlog order sequence and then added to the bottom of the backlog if and when the project becomes active again. This is even more critical with Dead project status. Dead is a final status and if those order numbers are tied to those tasks forever, then the order sequence will always be off.
REF Support Case 00409919