When a record is created and subsequently deleted in MongoDB, the transactions are stored in the oplogs. Because of this, when Fivetran captures the transactions during the sync, Fivetran will process both the insertion and deletion, even if the insertion and deletion happen all between 2 syncs.

This is not ideal behavior for MongoDB tables that have a lot of insertions and deletions, as it causes a lot of extra MAR and rows in the destination when that data isn't even wanted in the first place.