Hi,
This was a spec bug in the 4.1 specification and was subsequently fixed in later versions of the OpenGL specification. The current OpenGL 4.3 specification says:
The number of isolines generated is derived from the first outer tessellation level; the number of segments in each isoline is derived from the second outer tessellation level.
I don't believe the fix was back-ported to the older version of the specification. Our implementation matches the 4.3 specification. We would recommend always referring to the most recent version of the specification, even if you are writing an application designed to run on older hardware or drivers.
Thanks,
Graham