The scale heavily depends on the number of items the seller has sold during the previous 12 months. My sister always had perfect 100% feedback, until she sold one cookie jar that broke in shipping. Buyer negged her without even contacting her about the problem, which knocked her rating down to about 96% instantly.
If a seller does only 20 sales per year, one undeserved negative turns
100% into 95%. 15 sales per year, one negative turns 100% into 95% 10 sales, one negative turns 100% into 90% ... 2 sales, one negative turns 100% into 50%