Target $18.72, page-70

Because there's a limit on the price they can purchase at. If it goes up too quickly they can't buy shares so therefore if it rises fast for a sustained amount of time they can't purchase and they could potentially run out of time, resulting in them not purchasing all the shares they could have purchased.

Also, if the price falls they can buy back shares at a cheaper price which is good as that means they will use up less cash (hence leaving them with more cash on their balance sheet than they would have otherwise recorded which is a big plus).

in the long run, share prices will reflect the increased EPS due to the buy back and the price will increase. A short term fall does not mean the price will remain low forever.