What don't you understand? The Wrights figured 66%, yet the team achieved 75 - 82. When I did maths 75 - 82 was greater than 66. That is, the props were more efficient than the Wrights believed with their 66%.
The Wright brothers did not just believe or just calculated their propellers might be 66% efficient. They built them and measured their efficiency as being 66%.
In the article "The Wright Brothers' Aeroplane, O. and W. Wright, The Century Magazine, September 1908, pag. 648-649,
The Wright Brothers' Aeroplane [Orville and Wilbur Wright, The Century Magazine, September 1908] | Library of Congress ", the Wright brothers wrote:
"Our first propellers, built entirely from calculations, gave in useful work 66 per cent. of the power expended. This was about one third more than had been secured by Maxim or Langley."