After a sprint is completed stories are carried over where work has already been done. A 13 might have 5 story points work done already and 8 points left. If an additional 5 points of work are done and the story is carried over again, then the final sprint for that story will have 3 story points left.
So:
- sprint 1: 13 points estimate
- sprint 2: 5 points estimate
- sprint 3: 2 points
Are the reports of previous sprints modified every time the story points are amended?
If so, that should be a bug IMO as it messes with sprint velocity and forces users to keep track of the actual story points in some other manner, which introduces an easy way to make mistakes.