Some values that Facebook calculates as unique — such as reach and conversions — can return different values in Supermetrics reporting than in Facebook Ad Manager when they’re part of a Supermetrics query in Data Studio (formerly Looker Studio) that uses filters.
This happens because Data Studio pulls the full data set and the filters for it post-query, leading it to sum the remaining values. Stay up to date on Google’s response to this issue in its bug tracker.