because of the reflectors, which run parallel to the bulb. In some situations, the footprint of the light from the reflector fits the tank better perpendicular to the tank length.
The bulbs do not provide a point source. They are dark at the ends of the capsule, obviously. So, if you want a good spread, orient the capsule so that is perp. to the long side fo the tank. Just envision a model emitter that is a cylinder. If you orient it parallel, then most of the light will "shoot" out the front and back of the tank.
The bulb inside is a cylinder, though small - it's producing an arc which is a linear source of light, not a point source. So the highest intensity is on rays perpendicular to the arc itself. Plus you can space bulbs much farther apart in this way - I've got a single 400W bulb over my 48" tank and I've got acropora growing on the far corners of the tank.
Yup, halides produce more light out the perp side, then the || side. You may not realize but because there is more light spread when mounted perp, if you mount two close enough together where the light converges from two light sources will result in twice as intense peaks in the middle. But, best way to demonstrate this is to try mounting them || then mount them perp and go with the configuration that lights your tank best.
