The likely cause is due to the Azure and GCP Cost History reports displaying your costs in the currency that your Cloud Provider bills you in where the platform may be converting to US Dollars.

For instance, if you are billed in Euros the report will display cost in that currency while the platform may display the cost in USD.  In order to best allow you to compare your Azure and/or GCP costs against any AWS spend, the platform converts these costs into USD ($) amounts in the Multicloud Cost report so be sure to take that into account when comparing totals from each source.