Some of it has to do with the age and condition of the tank and the nozzles, for lack of a better term. It can affect the depth and / or thickness of the ribbons, for example, or whether a patch is just on the surface or extends a little into the marble. Tanks, basically made of fire brick, wear out / wear down every few years and need to be rebuilt or replaced. The funnels through which a secondary glass is introduced into the tank (near the "drain hole" through which the molten glass stream will exit the tank) can vary somewhat in size through age, use, and replacement as well.
The chunks of furnace brick or sand-like debris in some marbles? That's the tank deteriorating --