For the data in Exercise 12.2.7 there were two observations for which X = 0. The average

Question:

For the data in Exercise 12.2.7 there were two observations for which X = 0. The average response (Y value) for these points is 33.3 + 31.0/2 = 32.15. However, the intercept of the regression line, b0 is not 32.15. Why not? Why is b0 a better estimate of the average fungus growth when laetisaric acid concentration is zero than 32.15?