The way you use the car changes the way the engine 'loosens up'. I've had 2 WRX's (same year, age, km's, etc) side by side with exactly the same mods and mine produced ~10kW more ATW and more torque. Same dyno, same day.
The biggest difference is that I give my cars hell on a daily basis and the other car (a mates) isn't driven as hard.

The IC will make a big difference as will the mufflers (probably more than the IC) but there's more to the equation than just 'dyno chart' compares. ;-)

EIDT: Not to mention the affect of something as simple as tyre pressures. ;-)