Find the Mistake

Below, two pages from the book "Defects and Defect Processes (Hayes and Stoneham) are shown.
  Can you find the mistake?

Why is the result correct anyway?
 

Link to theSolution

Hayes and Stones p.1Hayes and Sones p.2

gehe zu Defekte in Kristallen: 2. Eigenschaften von Punktdefekten, Gleichgewichtsthermodynamik, 2.1

gehe zu Solution to Exercise 1.1