An iridium spark plug can reduce the fuel consumption. Maybe the guy who owned the bike before plugged one in and they changed it at the service to a standard spark plug again.

I experienced it at my Z. My bike used about a liter more with an almost new standard spark plug. After like 2000 km with the standard Spark plug i experience a tremendous increase of fuel consumption. After changing back to my used iridium spark plug that had already 8000km done the problem was gone. After 16000 km the iridium spark plug still works like a charm with 4,2 liters per 100km. But now it is time to change it to a new one .

Maybe u can also check if the ignition coil works correct. Maybe there is some partial short cut that occurs when the engine vibrates so there will misfiring ignition sparks. Or the plug of the spark plug is broken.