That is how ISO works. Less ISO means more light is needed to the same brightness. Like on a sunny day when all your pictures get burned in or you want to take picture of the clouds in the sky, you make pictures with less ISO. On the other hand when you are in a dark room, but do not want to use flash, or the room is too big for the flash to do any good, you take up ISO. This will also make your pictures more noisy, but that is the price you pay.