Database size diff between atlas information and listDatabases command

I have a M10 Cluster on MongoAtlas. In the Atlas Overview the Data Size is 6.6GB, but when I run the command:
db.adminCommand('listDatabases').totalSizeMb
through Compass once I have connected to the cluster, the returned size is about 4550 Mb (about 4.5GB). I was wondering why this difference. Thanks.