27% is high, but Phil Knight hasn't been doing this for long. In the last 12 months, Bill Gates sold 148.5M shares of Microsoft, leaving him with 641M shares. So he sold 19%.
He's been doing this for 10 years, timing sales to cause the price to stay stable and not drop. So long-term owners have seen no profit.
I figure 6 years from now he'll be out and the price will move.