Reading pseudo columns _FILE_NAME is not free in BigQuery contrary to the documentation

as per the documentation , see here https://cloud.google.com/bigquery/pricing ,

Reading pseudo columns You are not charged for querying the contents of the following pseudo columns:

_FILE_NAME — Used when querying tables based on external data sources

I run this Query and it did billed me 31 MB

SELECT _FILE_NAME as f FROM `xxxxxxx.yyyyyyy.scadaexternaltable`

what I am doing wrong ?

thanks