I am executing the du command and I am receiving permission denied on some directories.

Code:
linux158:/usr/IBMi/depo/tools/rpm # ls -l
total 8
drwxr-xr-x 2 root root 4096 Sep 15  2008 aix
drwx------ 2 root root 4096 May 16  2008 src
linux158:/usr/IBMi/depo/tools/rpm # du src
du: cannot read directory ‘src’: Permission denied
4       src
On another system I created a test with a directory having the same permissions but the su command worked!
Code:
root@linux140:/root #~>cd /mnt/
Wed Jun 07 2017 09:16 AM
root@linux140:/mnt #~>mkdir test
Wed Jun 07 2017 09:16 AM
root@linux140:/mnt #~>chmod 700 test
Wed Jun 07 2017 09:16 AM
root@linux140:/mnt #~>vi test/test
Wed Jun 07 2017 09:17 AM
root@linux140:/mnt #~>ls -l
total 0
drwx------ 2 root root 18 Jun  7 09:17 test
Wed Jun 07 2017 09:17 AM
root@linux140:/mnt #~>du test
4       test
Are there any suggestions to explain this problem?