The api ignores option_group_id when creating a new custom_field.

If you use the api to create a custom field, like

$api_result = civicrm_api3('CustomField', 'create', array(
'custom_group_id' => 10, // some existing custom group ID
'name' => 'voorkeursafdeling',
'label' => 'Voorkeursafdeling',
'data_type' => 'String',
'html_type' => 'Select',
'is_required' => 0,
'is_searchable' => 1,
'weight' => 2,
'is_active' => 1,
'option_group_id' => 92, // some existing option group id
));

it ignores the option_group_id, and just creates a new (empty) option group. You can work around this problem by updating the custom field after creation:

$custom_field_id = $api_result['id'];
civicrm_api3('CustomField', 'create', array(
'id' => $custom_field_id,
'option_group_id' => 92));