Fill Bucket with Contiguous on laves small unfilled line area.

I am trying to understand why the fill bucket leaves a small line of unfilled area when I use Contiguous.  Here is an example.  I start with a blank canvas.  Half way down I take a hard round brush and paint a thick line across the canvas from side to side.  Now I take the fill bucket, and the Opacity to 100 percent, Tolorance at 100 percent, and Contiguous checked.  I fill the area below the thick line with the same dark color.  But it leaves a small thin line of unfilled area between the thick line drawn with the paint brush and the filled area.  I then have to go and paint in the unfilled line.  Can someone explain why this happens.  I have tried setting tolorance to 1 or 100 and the results is the same.  Am I missing something?