I try to explain.
REQUEST_URI normally contains the current URL starting from "path". If an internale redirect happens (e.g. sh404SEF), Apache sets REDIRECT_URL, while IIS sets HTTP_X_REWRITE_URL.
For the second case, you have an option on Tag Meta plugin settings (REDIRECT mode).
But first I'd like to take a look at you Tag Meta items to sse how they are configured. Meanwhile, you can trace the code just adding a:
print_r($_SERVER['REQUEST_URI']);
print_r($_SERVER['REDIRECT_URL']);
exit;
Just to check how they are set on your site.
Regards,
Luigi