Wildcard Doesn't Match Using LIKE '%' On A Varchar Field, Wierd!

Right this has to be a Micro$oft mess-up surely...?

I'm running SQL 2k standard with SP3. I have a table which I'm trying
to query using a LIKE operator on a varchar field as follows

....
WHERE dbo.tbl_pm_projects.SeniorManagerID LIKE '%'
....

In actual fact the % is passed in by the application when the user
selects "All managers" from the drop down list used to select the
Manager to filter by. If they select a manager's name from the list
it becomes LIKE 'ajames' or whatever.

BUT - the table currently contains 2972 records. If I take out the
WHERE clause the SELECT returns all records - fine - but if I put the
where clause in it returns only 1682!! I thought the % was meant to
match, and I quote the SQL server Books Online files here; "Any string
of zero or more characters." Anyone explain to me whats going on
here?

TIA
Niall