In some (few?) cases it could be because there was something not quite right or outright defective with the device, and Apple presumably either tested or repaired it. In which case it may not be exactly equivalent to new.
They may not be "exactly equivalent to new" in some cases but they're pretty close and in some cases a safer bet than brand new. The new products only go through the standard quality check whereas the refurbs pass a more rigorous series of inspections.