To answer your question at a fundamental level, it is because Chlorophyll A and Chlorophyll B are different proteins. Every protein is made up of small building blocks called amino acids. There are only 20 amino acids that make up every protein in our world! All the variety we see in the world is due to the different types and number of amino acids in each protein. Therefore, Chlorophyll A and Chlorophyll B absorb different light wavelengths better because each type of chlorophyll protein is made up of a different combination of amino acids, which react differently to light.

On a more general level, though, why would a plant need to have two different types of chlorophyll to absorb different wavelengths of light? By having different types of chlorophyll that absorb light at different wavelengths, plants can use more of the light produced by the sun. If a plant can only absorb light at one wavelength, it can only use that one type of light to make energy. Imagine if people were only able to use carbohydrates, like bread and rice, for energy. We would be in big trouble if we couldn’t find this sort of food to eat. Having different types of chlorophyll that absorbs different wavelengths of light is similar to our ability to use carbohydrates, fats, and proteins for energy.