Hi,
Good question
In fact any path on a filer start with /vol/ - you can skip it though (in certain context) for volumes. So e.g. both vol1 & /vol/vol1 may be used, giving the same result for many commands.
As LUN sits within the volume, we have a longer path, i.e. /vol/vol1/lun1
Why though? I reckon it's just UNIXe nature of ONTAP - there is a root of file system (/) & all user data is in /vol/ folder.
Regards,
Radek