There is a previous thread where I noted that the problem we have is that the Canvas Grade is not, and will never be, the final reported grade/ GPA.
The simple rationale for that is that our Registrar needs control over average calculations and will not yield that to Canvas.
Additionally, we apply penalties and/or caps on marks according to lateness, misconduct etc. and these are applied on our SIS side rather than in Canvas.
You could clarify whether you are using Classic Quizzes or New Quizzes. It has taken four years for New Quiz activity (participation0 to be reported in New Analytics, and I've still found gaps between different views of what should be the same data.
If you are getting an anomaly it would be worth escalating it through your Canvas success manager, as this could have fairly serious ramifications.