Because they switch the placebo and creatine groups :)
Week (-∞)-0: Noone has taken any supplements
Test 1.
Week 0-6 : Group 1 gets creatine, group 2 gets placebo
Test 2.
Week 6-12 : Noone takes any supplements
Test 3.
Week 12-18 : Group 1 gets placebo, group 2 gets creatine
Test 4.