All custom attributes gets appeared under "additional_attributes" header in the export separated by comma (,). I am using magento 2.0.7.
[![enter image description here][1]][1]


  [1]: https://i.sstatic.net/Zi1Fi.png

To make these attributes occupy a separate cell, we cant add more columns to the CSV. Please refer https://magento.stackexchange.com/questions/130196/do-you-have-to-use-additional-attributes-in-magento-2-product-import-or-can-you/130203