The convention in science is that you display as many digits as your measurement is significant to. If it were significant to two digits, it would've been 99%; if it were significant to four digits, it would've been 99.90%.

In any case, that number's backed up by several sources:

http://abcnews.go.com/Technology/story?id=99478&page=1

http://news.nationalgeographic.com/news/2002/09/0924_020924_...