This is by design:
IBM needs to set this to Person when the names in the ACLs (Access Control List) are not distinct. For example, you put a name in the ACL which exists as person as well as a server name. While Lotus Notes is obviously able to handle this anyway, you now need to set the user type to Person.
One Identity Manager only deals with user accounts of type "Person" (disabled/inactive accounts are NN_Person, but those names were changed to NN_ as well).