difference between extruded aluminum and sheet aluminum

I would hazard a guess that extruded is one composition of metal, sheet is basically random and different. They are probably buying extruded based on that composition, as far as the mill is concerned. Theoretically, you could probably push average aluminum through an extractor and it still be base aluminum, not the chemical composition needed for true extrusion. An XRF gun and some research could probably answer this more definitively.