Never noticed this before until we posted a product at £10 (inc. 20% VAT) which was reduced to £5 (inc. 20% VAT), which displayed a banner saying Saving 45%.
I have changed the percentage calculation and it is working now.
Open /catalog/includes/modules/product_listing.php and /catalog/includes/modules/product_listing_col.php
$discount = (((($fullprice * 100) - ($saleprice * 100)) / ($fullprice * 100)) * 100);
$discount = ($saleprice / $fullprice) * 100;