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http://www.users.bigpond.com/darren.hurrell.htm

Trivia Fact: The leading 10 jockeys in Sydney have ridden over 80% of the winners in Saturday meetings in 2002.

That's why they are the leading jockeys.
If they didn't win most of the races, they wouldn't be the leading jockeys would they?

If the top 5 won 80% of the races - that would be significant - but top 10??. Doesn't mean much at all.