How can a LEFT OUTER JOIN return more records than exist in the left table?

Table1                Table2
_______               _________
1                      2
2                      2
3                      5
4                      6

SELECT Table1.Id, 
       Table2.Id 
FROM Table1 
LEFT OUTER JOIN Table2 ON Table1.Id=Table2.Id

Results:

1,null
2,2
2,2
3,null
4,null

The LEFT OUTER JOIN will return all records from the LEFT table joined with the RIGHT table where possible.

If there are matches though, it will still return all rows that match, therefore, one row in LEFT that matches two rows in RIGHT will return as two ROWS, just like an INNER JOIN.

EDIT: In response to your edit, I've just had a further look at your query and it looks like you are only returning data from the LEFT table. Therefore, if you only want data from the LEFT table, and you only want one row returned for each row in the LEFT table, then you have no need to perform a JOIN at all and can just do a SELECT directly from the LEFT table.