Real life is not ideal, so actual torque varies because such things as intake and exhaust-pipe resonance and airbox effects exist. They cause local variations in the height of the torque curve.
In the case of a traditional touring or cruiser engine, the torque rises to a useful amount at as low as 1,200 rpm, peaks somewhere in the usual range of 2,500 to 3,000, and then slopes gently downward after that. Why? This is the “running out of breath” effect mentioned earlier. As the engine revs up, there is less and less time for cylinder filling, so the short valve timing and moderate port sizes of such engines cause cylinder filling to become less and less complete as the engine revs up, so torque falls.