Check what David said.

A member of db_owner, except for dbo, will honor a DENY on a table. Members of the sysadmin role map in as dbo, which effectively ignores the permissions.

Execute this query in the DB to see what permissions are assigned against the table:

SELECT t.name AS 'Table', u.name AS 'User_or_Role', dp.state_desc, dp.permission_name

FROM sys.database_permissions AS dp

JOIN sys.tables AS t

ON dp.major_id = t.object_id

JOIN sys.database_principals AS u

ON dp.grantee_principal_id = u.principal_id

WHERE dp.class = 1

AND t.name = '**Your Table Here**';