Well... when the manufacturers make these modified replacement chips, they just take a mean average accross a pool of cars (often not many are used as a sample, unless they state this be careful) and set a slightly different set of values on the chip to get the engine pulling harder. I.e. they adjust the fuel mix, engine timing etc that they think will suit all.
This means that many off-the-shelf chips arent always the best route to go - yeah you may see some gains, but as each engine is always different you cant guarantee that an off-the-shelf chip will be totally suited to the unique properties of your own engine (wear, tear, breathing, etc) - even cars of the same model/spec etc will have engines which behave differently, so ideally you wanna go for a professional custom map...
This means giving someone your car for a day, them hooking it up to a rolling road etc and spending at least whole afternoon adjusting the values to obtain the best torque/bhp curves...
But what do i know