Google Adwords algorithm

Interesting experiment to test the impact of changing the rate for Google Adwords. Two identical sites have Adwords account. One has an established account paying $0.10 per word. Another is a new account, and begins by paying $1.00, then lowers it to $0.40. The former got 15,000 click-throughs regularly. The latter started just above 15,000 and fell to 1,200 after the rate drop. Does the Adwords algorithm favours rate increases disproportionately (and hence is evil?) Or is the fact that the former site an established one create the difference?