I would guess that you don't have sufficient permissions to see the remaining data.
For sys.database_principals, Books Online says:
Any user can see their own user name, the system users, and the fixed database roles. To see other users, requires ALTER ANY USER, or a permission on the user. To see user-defined roles, requires ALTER ANY ROLE, or membership in the role.
And for sys.database_permissions:
Any user can see their own permissions. To see permissions for other users, requires VIEW DEFINITION, ALTER ANY USER, or any permission on a user. To see user-defined roles, requires ALTER ANY ROLE, or membership in the role (such as public).
What you describe sounds quite much like a user that has none of the permissions mentioned above.