Your connector is not tracking the changes that you make in the source files as changes and is instead syncing the rows as if they were new every time.
Connector: File connectors (except Amazon S3 and Azure Blob Storage)
This is expected functionality. Please see the Cause section for more information.
File connectors (except Amazon S3 and Azure Blob Storage) do not track changes at a row level. This means that all rows count towards MAR every month.
You can read more about this in our documentation section Consumption Based Pricing - File Connectors.