It had to do with the pattern on the face of the bulb lens. The pattern on the face will focus the light as it passes through the glass. I'm hoping someone finds a good picture of the two different styles side-by-side and post it. The narrower/tighter pattern that they developed later on would focus the light a little farther out and you might see them advertised as +40 bulbs because they would supposedly give you 40 extra feet of driving light...