I've tracked this down to a "bug" or oversight in the /usr/local/directadmin/scripts/user_to_reseller.sh script.
It does not check the DirectAdmin config variable reseller_allocation_include_self and assumes that reseller allocation will include self, so it prefills /usr/local/directadmin/data/users/%reseller%/reseller.allocation with disk and bandwidth already assigned to %reseller% when %reseller% is converted to a reseller.
This apparently gets all sorted out when the daily tally runs - action=tally&value=all - although, I'd prefer something that can rebuild the /usr/local/directadmin/data/users/%reseller%/reseller.allocation file without having to run tally for all users. But I haven't figured out what that is, or even if such a command exists.
It would appear that moving another user into a reseller doesn't trigger a rebuild of the /usr/local/directadmin/data/users/%reseller%/reseller.allocation (either through the /usr/local/directadmin/scripts/move_user_to_reseller.sh script or the CMD_MOVE_USERS API). So this needs to be done manually after moving an account to a reseller. I can script something to do this, but I'm wondering if there is a more eloquent way already built-in to DirectAdmin to accomplish this, since I know action=tally&value=all will do this, I just prefer not to run tally for all users after creating a reseller.