The quiz was made in 2011 and it was updated multiple times. On sporcle answers added after the creation of the quiz will never be at 100% because it tracks every play even the ones before the answer was added.
Steve Kerr told a story about how old he was while playing.. I think in San Antonio.. said a fan yelled "hey Willis, you single? My grandma kinda likes you!"
He is in there a bunch of times, i'm guessing that the answers have been updated multiple times and it takes it as if the people who took the quiz before didn't knew the answer