The answer is RPM.
The stock torque peak is low, 2800ish, then holds out for 1000 or so and falls off a cliff because the stock exhaust can’t flow enough at higher RPM.
All these after market header setups are taking advantage of two things, flow and pulse tuning.
Flow - they all use bigger than stock diameter pipe so they can flow more, holding that ~1000 RPM torque plateau out a bit longer, maybe 1500.
Pulse Tuning - By using proper length primary tubes they’re engineering in a exhaust pulse resonance at 4000-6500 RPM depending on which system it is. I’ve done the most math on CES, and they’re pulse tune event occurs right around 5800 RPM. As a result you’ll see a kind of double peak on the dyno sheets, the first peak is a result of cam timing, the second is a result of pulse tuned exhaust. The result is a wide extension of the existing torque to higher rpm.
Since HP=torque * RPM / 5252 keeping the same torque bit extending it up the RPM range results in a big increase in HP.
Taken the extreme you’ll see the Carpenter kit has a torque plateau that spans over 3000 RPM. That’s the result of complementary CAM timing and exhaust tuning, what you can’t see is that it still make very good torque down low, the result of the sheer displacement of the R3 motor.