Question
Why are some permissions set to 751 (drwxr-x--x) instead of 755 (drwxr-xr-x) on some directories even though 755 are the default permissions.
Answer
The permissions on a Unix/Linux file system as separated into three different regions. The first is the user region, the second would be group and the 3rd is called world. The world permissions is what's set for any user on a Unix/Linux server and why this is more strict.
In the example of the permission, bits separated this would be the following for 751
drwxr-x--x
The d at the beginning just means it's a directory.
Then the first three permission bits are rwx in parenthesis is the user portion which means the user can read, write, and execute
d(rwx)r-x--x
The second set of 3 bits is r-x and this is the group set of permissions and this is set to read and execute.
drwx(r-x)--x
and then the last 3 bits are for the world or all users
drwxr-x(--x)
and this is set to executable only which restricts just about everything for any user unless they are a user or belong to a group that proved permissions at that level.