I'm not 100% sure, but I think turning the volume up is actually turning the attenuation down. If you were at -45, you turn the volume up, which is attenuating the signal less, which is why the number goes down towards zero, and the volume gets louder. I think the number you're seeing is dbm, which is what attenuation is measured in. I may be totally off, but I think I'm somewhat right. I work in intermediate level navigation systems for aircraft, and what I said is how we measure signal strength in certain systems. To your question, it's perfectly normal. If the number went up when the volume went up, that would be a problem. Hope I helped.
Joe