different records result if a use N (unicode data)

A same query with a NOT LIKE statement and a wild character % returns
different records result if a use N (that means that the string follow is
unicode data) or not.
Par example:
select * from company
where company_name not like N'%'
select * from company
where company_name not like '%'
the result of the two queries is different.
How is it possible?
Company_Name is a varchar field (not a nvarchar)
RegardsI forget this happen if there are at least one record company with
company_name NULL
Thanks