Why Parameter Inclusion Causes Less Records????

My question is why does the inclusion of my parameter, @GL, cause my report to only show records where the LEFT OUTER JOIN to the GRPLDR table is NOT NULL from tblMain.

Basically, with the parameter in the report, I get 6 pages. Without the parameter, I get 10 pages and those records where the GRPLDR.GLPerson is null for the join.

I tried the exact same query is Crystal reports, and get exactly what I needed by the "NULL values set to default" option.

Here's my query:

SELECT field1, field2, GRPLDR.GLPerson
FROM tblMain LEFT OUTER JOIN
(

SELECT Person AS GLPerson, WPIdentifier
FROM SigAuth WHERE (Title = 'Specific Value')

)

AS GRPLDR ON tblMain.WPIdentifier = GRPLDR.WPIdentifier
WHERE (GRPLDR.GLPerson LIKE @GL + '%')