Multiselect values not getting saved in hook_settings()?
1) If variable_get('post_type_unique_serial') returns an array to populate the options, then it can not also be used for setting the default value, as this should be a single key within the array.

2) Generally it is good practice to use the same name for the variable as the form element if it is being used to set the default value eg $form['MYMODULE']['post_type'] and '#default_value' =&gt; variable_get('post_type'),