Programming Machine Learning: Bias in linear regression

Hi there,
bought “Programming Machine Learning” (great book) from pragprog.

A question (from a newbie) for the author, or anyone can help:
At the end of the chapter 4 in the paragraph “Bye Bye, Bias” is described how to add the bias parameter to the multiple linear regression algorithm.
Why the bias is maintained constant at every iteration when in the “single” linear regression in chapter two it is variable?

Thank you!