I know this is an over-generalization, but often "tunes" are adjusting throttle response to provide more power with less movement of the accelerator pedal. This gives a big improvement via butt dyno, but may show little to zero power gain on a dyno (or at a dragstrip), at least for normally aspirated engines. Forced induction engines generally get the same throttle-response bump, along with more boost, to provide real gains (pulley swap needed for supercharged applications). As mentioned above, a lot depends how much R&D was put into the tune. Sometimes, sadly, the customers are the beta testers. Especially for low-volume engines/vehicles.