Highways are a result of crossing gravel and yellowbelly, while mystic potions have the mojave and mystic genes. So, nope, they don't have any of the same genetics! The simple answer is that not all mystic potions have a perfect, unbroken line down their back. Many have this more "dashed line" appearance. There are some pictures of other mystic potions on World of Ball Pythons if you want more examples: http://www.worldofballpythons.com/morphs/mojave-mystic/