The same inverter 600W, load the same electrical appliances, why the output power will be different?

The same inverter, load the same electrical appliances, why the output power will be different? One of the reason is the surface clip connect to the battery .Pls see below two click https://www.youtube.com/watch?v=i5qtO7Mj27E & https://www.youtube.com/watch?v=2GLz-DOSMXs