difference between the values appear in report output on FA and policy count on FG

I want to monitor bandwidth used by Fortigate policies with Fortianalyzer but there is difference between the values appear in report output (bandwidth policy:Top 30 Policies by Bandwidth) on Fortianalyzer and policy count(byte in 5.4) on Fortigate. (I. e. the policy count is 58.55MB and the Fortianalyzer report shows 55.84MB for bandwidth policy) I'm confused which one is right. What does the difference mean? How can I get the correct result?