Why are stratovolcanoes sometimes called composite volcanoes?

The cone is formed by alternate layering of lava flows and ash (or cinder) emissions. This means the type of eruption can vary from one episode to the next; from a relatively "quiet" extrusion of lava, to a more eruptive type emitting ash, gas, and steam. So, it's called a stratovolcano because of the characteristic layering, as much as it is called a "composite" volcano for the same reason.