Chemistry question about the reactivity of non metals!! please help?

The question says "Reactivity of non-metals decreases down a group. Explain your results from question 2." My results from question two that this is referring to were: The higher the halide is in the group on the periodic table, the harder it is to oxidize (react with hydrogen peroxide or potium permanganate). The halides I used were iodine, bromine, and chlorine. Chlorine is the highest on the group, followed by bromine, and iodine is the lowest. In the experiment I acidified each halide with nitric acid and added potium permanganate. I acidified each halide again and added hydrogen peroxide. The iodine had the biggest reaction and the chlorine had the smallest reaction. So my question is if the reactivity of non-metals decreases down a group, then why did iodine (which is below bromine and chlorine) react more than bromine, and bromine (which is below chlorine) react more than chlorine?