audio level as shown in clip

This may seem a small thing but it's annoying me, and I know there are other ways of adjusting the audio level. When you put the mouse pointer on the audio of a clip it shows the level as 0dB. If you move it up or down it changes the level. My problem is, having altered the level in this way I can't get it back to 0dB, the nearest it goes is +0.54dB or - 1.15db. Why can't I reset it to 0dB?