executing query of cloned view affects original view

While working on views_calc (http://drupal.org/project/views_calc) i noticed some strange behaviour.

In pre_render, views_calc clones the view and executes some special query to execute aggregate functions (SUM, COUNT, ...). During this aggregation, views_calc removes paging on the cloned table and calc the resulting aggregation data.
This execution of the cloned view also affects the pager of the original view - it suddenly disappeary.

Deactivating the execution of the subquery leads to an active pager again.

Find the snippet that causes the issue:

  function pre_render($results) {
    parent::pre_render($results);

    // If there are no calc fields, do nothing.
    if (!$calc_fields = $this->get_calc_fields()) {
      return;
    }
...

    // Add grand totals to the results.
    foreach ($calc_fields as $calc => $field) {
      if ($summary_view = views_get_view($this->view->name)) { // <<-- views does a custom semi-deep copy here!
        //$summary_view->set_display($this->view->current_display);
        $summary_view->set_arguments($this->view->args);
        $summary_view->pager['items_per_page'] = 0;
        $summary_view->views_calc_calculation = $calc;
        $summary_view->views_calc_ids = array();
        $summary_view->views_calc_sub_total = FALSE;
        $summary_view->is_cacheable = FALSE;
        $summary_view->preview(); // <<-- this execution affects the pager of the original query (in $this)
        $this->view->totals[] = array_shift($summary_view->result);
      }
    }

It seems to me like views doesn't clone enough in views_get_view (finally $view->clone_view()).
What part do we miss here?