The permission model is designed such that you cannot see any Grants but that of yourself and/or of objects you own.
Scenario A: I am a "vanilla" user who is allowed to create a table in a schema. When I do SHOW GRANTS for my user ID, I'll see all the grants that "I" have (ie, objects I created + other grants I may have been given by other creators)
Scenario B: I'm a Schema Owner and execute SHOW GRANTS on SCHEMA "my-schema-name", I'll see all the grants "anyone" has on my Schema. (Similar for Catalog Owner).
This is why a Metastore Admin sees all GRANTS.