more columns = fewer rows?

I'm doing a pass-through query on an as400 from SQL Server via a linked server.  The more columns I include in my select query, the fewer rows I return.  How is this possible?

If I select:

column 1    i get 1400 rows
column 1-3 i get  874 rows
column 2-4 i get 834 rows
column 6-9 i get 763 rows
column 1-9 i get 41 rows

what am I going wrong?