It is a known issue, the reason is explained well at
Bug 26334 – offline: label in gmail should be displayed as a tag in ZDesktop instead of as a folder
I quote from Jeff Flanigan as you cannot see it, "This is a result of Google's unfortunate use of folders to represent labels.
They might have done better to use keywords instead so 3rd party clients would
be able to use labels in a standard way. The problem is compounded by the fact
that Google allows labels to act like folders as well as tags; a message may
have 0,1, or many labels. If it has just 1 label and is not present in the
inbox it is considered to be 'in the folder' represented by the tag. "