I thinks it's to offer protection to Dettol manufacturers. It is common to see all product in the category or disinfectant or other medicated product use 99.99% as their marketing gimmic. Germs can cause infection or disease. read more
You also might've never used Dettol but think you can sue them for any sickness that falls under the Sun just because they claimed to kill 100% germs. But when they say 99.99%, they're just being safe against a possible germ.. read more