Referencing this post by Blake - http://greensock.com/forums/topic/15152-drawing-in-a-mask-doesnt-completely-cover-line/#entry65724
It would be awesome if we were able to animate stroke width across the length of a path! Blake mentions that in the process you might be able to fix a browser masking issue we have no control over where there are overlapping strokes in a path due to higher stroke-width values.