Reflector design can dramatically impact the amount of light reaching your plants. In the diagrams pictured (assuming they are all using a 1000W lamp) you can easily see how three different reflector designs can change the focus and intensity of the light.

Another way to think about this focusing effect is to compare it with adjusting the spray pattern of your garden hose. Without a hose head, the water does not have guidance on how to exit the hose. With a hose head, you can focus the water spray and shoot it out of the hose in a variety of streams or spreads.

Example 1 shows the light as more intense but only covering a small portion of the plants.
Example 2 shows the light covering more of your plants but dropping slightly in intensity.
Example 3 shows the light expanding beyond the plants allowing for wider coverage but with the least amount of intensity.
Example 3 shows the light expanding beyond the plants allowing for wider coverage but with the least amount of intensity.

The quality of the material used in your reflector can also dramatically affect the intensity of the available light which hits your plant canopy. Lower quality materials are not as reflective initially and degrade quickly, which reduces the amount of light reaching your plants.