When you have 10,000 feed backs at 100% and you get a negative , it will not show up in the percentage. A person with 10,000 feedback can have upto 5 negative feedbacks and still be 100%. Ebay rounds up the percentage into a number with 2 decimal places.
Best policy is to list a lot of items , give good customer care and if you still get a negative out of the blue , contact services like squaretrade.com to mediate and remove the feedback for you.
Regards
Onesimpleman