I'm not exactly sure what caused the "separation" of the beams here, but I think it has something to do with the reflections from the water surface, and the camera angle. The top picture is a volumatric quadratic spotlight with a cone angle of 7 and a power of 400. The bottom one is a couple of Vue cone primitives with that "Glowing Moon" material from the "Glowing materials" library applied to them. They look very similar, but the one with the glowing materials instead of the volume light rendered alot faster. Glowing materials are good for things like this, but the down side is that you can't do a "resume render" with a glowing material in your scene.