Its input vs. output. How much its pulling from the wall vs. how much wattage output. But even then it varies by how much load its under.
Basically. The rating is in DC wattage though so at the 500w max with 75% efficiency it'd be pulling ~667w from the wall.So let's say its at full load. It would take 500W from the wall, and would output 375W combining all rails?