To make sure that your contact Custom Variables will be included when you Export your Contacts list via the Contacts Smart Table, you will need to make sure that you have added the custom variable in your account's Variables list.
To do this, go to Settings cog > Contacts > Contact Custom Fields.
Add the custom fields that you would like to be included here at the bottom of the page. Then make sure to click on the checkbox beneath 'Show in Search' so that information added to this custom field in your Contact profiles will also be included in your CSV Export.
Note: The ability to export information can only be accessed by Admin user types.