Bugs in Oracle Query ?

I want to query a data with date condition. Guess what ? This similiar query give a different result.

1st query give the exact result that i wanted.
SELECT * FROM my_table
WHERE TO_CHAR(date_time, 'DD/MM/YYYY') BETWEEN '01/10/2007' and '6/11/2007'

2nd query result nothing
SELECT * FROM my_table
WHERE TO_CHAR(date_time, 'DD/MM/YYYY') BETWEEN '01/10/2007' and '06/11/2007'

The different is, the 1st query use 6-11-2007 instead of 06/11/2007.

FYI, i use Oracle database 9i and running query in Toad for Oracle 8.5.3.2

Any idea why this can happen ?