OK, I am really confused. Looking at your CI graph, you were plateaued at 60 from May until early July of 96. From that, I'd guess you added a new campaign in lat June/early July, so around week 26 or so of 96. That was enough to start you climbing, up to a CI of 70, where you plateaued again. So from week 26 until week week 36, you had steady CI growth.
If the week 28 '97 marketing costs are more than the week 22 '96 marketing costs, then that'd make sense. Added enough to reach 70 plateau, were still at enough to reach 70 plateau despite the 20% decrease.
But if the week 28 '97 costs (good for staying at 70) are lower than the week 22 '96 costs (good for 60, not good enough to move towards 70), then that's weird. If you didn't add a campaign back around week 26 of 96, then that's even weirder.