I agree with Maxwell.
A lot items of performance figures are just one single item against an unmodified angine.

For example:
A new CAI may let the engine breath better and gain you 5hp.
A new exhaust may let the engine breath better and gain you 5hp.
However, if you do both mod's the engine can not breath the same increase twice, you may only gain 8hp with both mods.
The engine is only a glorified air pump, it can only move so much volume of air - hence turbo's & blowers etc.