There\'s a difference between sales as in raw numbers, and market percentage for a week. Sales = raw volume numbers. % = dependant on what\'s out there. You could have 50% of a really slow market and it\'s not as impressive as 25% of a stunningly hot week.
WOW sold 200K copies first day, over 500K after a week. Even games like Unreal Tournament won the % race, but it\'s numbers weren\'t that impressive (cause of all the crap available at the time).
Now, if that 13% marketshare is of a week where lots of PC games sold well, then yes, that\'s an impressive amount.