I wouldn't call it buggy, just counterintuitive.
Just remember that when you change volume, it doesn't REMOVE the previous volume and then put on the new volume, it just draws the new volume OVER the old one. And when you draw a new picture, the transparent pixels don't get drawn at all.
I've been musing on ways to take advantage of this for a really cool, original volume bar, but so far I'm coming up blank.