Normally if dust is powered through a block it looks like a circle and will power the adjacent sides. When the line is extended, it straightens out and no longer powers the sides like shown below. Now if the dust goes up a block it has the image of being straight, but isn't always just a line. For solid regular blocks things are fine. But with transparent blocks like glass, half slabs, etc. it keeps the sides powered. In the pictures below are examples of this happening, where even though it is a line, it still acts like the circle.