This is because you have added story points values to stories after the sprint has started or you have added new stories to the sprint mid-way. Sprint report will only take into account the story points for the initially committed scope.
A feature request GHS-8776 is already logged to show the actual completed scope rather than the initial committted scope.
A workaround can be to use the burndown chart instead.
Also have a look at the link below and the answers discussion on this topic.
...It's true that there are projects in Jira; but they are merely a way to cut off issues, to tell them apart from other sections of work and to apply rules that are specific to that team (the schemes)....
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