Perhaps its because the mapped drives are allready connected and authenticated, when you open a UNC share it has to login (if it isnt allready) then open a connection to what your trying to get to, or ...

I've got Win 8.1 and a network drive. I can see the network drive fine from Windows, but not under Cygwin. I get the impression I need to edit my /etc/fstab file to see the drive. But nothing I see ...