Maximum gains aren't always advertised as gains at the peak of the power curve. They can be advertised as the largest gain at any point in the curve.
When tuning for the Boss 302 intake manifold the rev limit is usually extended to 7500-7600 RPM. The factory rev limit for base GT engine is well below that. The RPM limit can be extended out using the OEM intake manifold but power will be falling off where the Boss 302 manifold is still able to make power.
This means that the OEM intake and factory tuning might make 375hp at 7300 RPM but the Boss manifold and aftermarket tuning might be making 435hp at that RPM. That's 60 hp, however it is "only" 15hp above the factory rating for the engine.
Not every place does this, but many do. Big numbers sell.