Well, from my understanding, the unset value allows the div to fill up whatever space is containing it, like water in a bottle. If it weren't for the wrapper, the div would fill the whole screen.
I do understand that this seems a bit odd. Usually you would set width:"100%", I used "initial" and "inherit" as well, and it would accomplish the same thing. But those values cause similarly frustrating malfunctions, namely resizing the initial width and height of the retracted div after the animation has been played and reversed.
One way or another you are spot on with my issue. It seems that the browser is taking some values and not refreshing them accordingly, no matter which way I am trying to accomplish this.