Why cronjob fails to execute script in 'sh' shell in Red Hat Enterprise Linux 6?

Issue

• Our cron job uses 'tee' command to write some messages into the standard output and a log file.
• When it's defined as below, it stopped on the way to output messages into a log file.

20 21 *  *  * sh /root/test.sh 

• But, when it's defined as below, all messages were output into the log file properly.

20 21 *  *  * sh /root/test.sh > /dev/null 2>&1

• Why it didn't work properly in the former case?