Why is energy lost from the 10% rule?

when the energy moves between each trophic level 10% of the energy is made available for the next level. this energy is originally from the sunlight. so if a primary consumer has 10,000 energy then the primary consumer would end up getting 1,000 energy. the secondary consumer then gets 100 energy and the tertiary consumer gets 10 and so on. hope this sorta helps