Hi @AmitG and welcome to the MongoDB community forum!!
The two variables $minCompatibleVersion and $currentVersion are deprecated schema version versions for config server and they are different from Feature Compatibility Version.
With the introduction of of Feature Compatibility Version, these variables are no longer used and are scheduled to be removed at a later date.
The server ticket https://jira.mongodb.org/browse/SERVER-68889 mentions the removal of these variables in the near future.
Let us know if you have any further queries.
Best Regards
Aasawari