How to delete a variable during uninstall? I've never run into this problem before, but the variable I set in my module won't delete during uninstall. What am I doing wrong?

I set the variable in my module like this:

variable_set('marigold_nutrients', $nutrients);


All is good. The variable saves just fine and I'm able to use it as normal with variable_get.

Here is how I try to delete it:

In my module's .install file, I implement hook_uninstall...

/**
 * Implements hook_uninstall().
 */
function marigold_nutrition_uninstall() {
  variable_del('marigold_nutrients');
}


Using the UI, I remove the module then uninstall it. The database table the module creates deletes just fine, and it looks as though it's uninstalled. However, when I look at the variable table in Sequel Pro, the 'marigold_nutrients' variable is still there. I've run cron, cleared the cache... not sure what to do. 

MORE INFO:
- Using the debugger, I see that when I uninstall the module, it steps into the uninstall function just fine, so I'm pretty sure that function is working.
The problem is probably here (from the comments):


  The variable gets set during installation when the hook_schema calls a function to iterate through an array to create database columns.


The uninstall process calls drupal_uninstall_schema(), which in turn calls drupal_get_schema_unprocessed(), which invokes hook_schema().

So the variable is being added again during uninstall, presumably at a later point in the process than your hook_uninstall() is invoked and variable_del() has run.