Lots of possible reasons for this. The best way to start IMO is to disconnect the extruder from the hot end and extrude something like 100mm of filament and measure what you actually get, then adjust the steps per mm accordingly. It should be in the region of 492.45 steps per mm according to the link. Then reconnect the hot end and try again at a low speed. If you get significantly less, then it's something to do with pushing filament through the hot end and not the extruder steps per mm. Maybe skipping steps or grinding filament because of a partial blockage, or motor current set too low, or hot end temperature too low for the filament etc.
