Odd issue with offset?

Trying to make some nice isocurves for machining, but having some odd issues with grasshopper and the offset parameter.

Before as you see all is fine, but then doing the offset it acts up.

Depending on the slider settings it misses differens offsets of the curves, where some are missing now some shows depending on the value on the slider.

Before offset:

http://img441.imageshack.us/img441/1669/preoffset.jpg

After offset:

http://img405.imageshack.us/img405/1174/postoffset.jpg

Any clues? :)