There's any number of things that contribute to (or detract from) the consistent production of torque in an internal-combustion motor, and the presence of some sort of exhaust resonance is only one of them. You've got intake resonances and airbox effects, intake restrictions, cam timing, ignition timing, backpressure, flywheel effect...a few years' worth of physics classes all competing and boosting each other. (For the downside of this, try to find a copy of the current "Bike" magazine, and the parts about the old ZXR750's severe power problems around 6000 rpm.) A mild hump like that is probably due to a slightly better-than-average confluence of a few of these, but it's probably going to be unnoticeable, and it's still significant that on the whole torque output is so smooth, giving away nothing at either end of the rev scale and avoiding sags or holes in output.