Sub theme template.php not overriding

I'm using the dev version of the latest Zurb foundation theme, but I don't think it is a theme specific issue.

A few variables for column classes are set in PARENT_THEME_preprocess_page() along with some other stuff. If I try to override in my sub theme below it doesn't apply.

However, if I comment out the corresponding section of code in the parent theme ( which is obviously bad practice) it does work.

Here's my sub-theme template.php code:

function THEME_preprocess_page(&$variables) {

  // Convenience variables
  if (!empty($variables['page']['sidebar_first'])){
    $left = $variables['page']['sidebar_first'];
  }

  if (!empty($variables['page']['sidebar_second'])) {
    $right = $variables['page']['sidebar_second'];
  }

  // Dynamic sidebars
  if (!empty($left) && !empty($right)) {
    $variables['main_wrapper'] = 'large-13 large-push-3';
    $variables['main_grid'] = 'large-10';
    $variables['sidebar_first_grid'] = 'large-3 large-pull-13';
    $variables['sidebar_sec_grid'] = 'large-6';
  } elseif (empty($left) && !empty($right)) {
    $variables['main_wrapper'] = 'large-16';
    $variables['main_grid'] = 'large-11';
    $variables['sidebar_first_grid'] = '';
    $variables['sidebar_sec_grid'] = 'large-5';
  } elseif (!empty($left) && empty($right)) {
    $variables['main_wrapper'] = 'large-12 large-push-4';
    $variables['main_grid'] = 'large-16';
    $variables['sidebar_first_grid'] = 'large-4 large-pull-12';
    $variables['sidebar_sec_grid'] = '';
  } else {
    $variables['main_wrapper'] = 'large-16';
    $variables['main_grid'] = 'large-16';
    $variables['sidebar_first_grid'] = '';
    $variables['sidebar_sec_grid'] = '';
  }
}

Any ideas?

Thanks,
Chris