System Stored Procedures/behavior Not Understood

I recall that stored procedures created in the master database and having a name beginning with sp_ are available from all databases. However, I've found varying results when invoking such a procedure from a different database. For example:

DECLARE tables CURSOR FOR
select name from sysobjects
where type='U'
order by name
for read only

populates a cursor with the list of tables in the database from which you are running the procedure. However this query in the same stored procedure:

SELECT count(*)
FROM sysExcludeMaint
WHERE @tablename like tablename

fails with "Invalid object name 'sysExcludeMaint'." if the table sysExcludeMaint
doesn't exist in the master database (it does exist in the database from which I've invoked the proc). I'm not clear on why the different results. Anyone know?