Long standing discussion, and have many theories.
1. Direction Injection switching
2. Timing retardation to work with DI
3. Intake Acoustics
4. Non-optimized AVCS
5. Combination of 2 or more of the above.
Personally i believe its the result of DI...
The IS350 with similar injection has similar TQ fluctuations when switching to DI for relying on intake pressures and higher cylinder compression to form a homogenous mixture.
as well as the GS350
While just looking at this curve may turn you off, its actually the best way to make this much power while using the least amount of fuel and pumping out the least amount of emissions. While a similar conventional port fuel injected system can make the same power or more...it will also use more fuel and not pass our ever stricken global emissions laws. Even the Civic R FN2 has been discontinued due to not passing emissions in Europe and it makes a similar powerband that of the FA20.