Currently balancing threshold are not clarified and the section contains a link to itself:

https://www.mongodb.com/docs/manual/core/sharding-balancer-administration/#std-label-sharding-migration-thresholds

Also, the "migration thresholds" link points to the same section - that should be fixed/removed.

Explanation provided by engineering
The migration threshold for 6.0 is 3x the configured chunk size for a given collection. So for the default chunk size (for 6.0) of 128MB, two shards must have a data size difference for a given collection of at least 384MB for a migration to occur.

At steady state it's ok to have shards' data sizes slightly differing. Much better than continuously moving data to reach perfect equality.