usermod and groupmod inconsistency for removing multiple users/groups

I don't understand why usermod has a -r option that can be used in combination with -G to remove groups that the user belongs to, but groupmod doesn't have an option to remove multiple users from a group. Both however have -a to append users / groups. Is there a reason for this inconsistency? Is it just me, or some of these commands are poorly made and documented? I have to spend some good time figuring out some things.