Interesting. In my real world example above re: Honda/Acura that still doesn't make much sense to me though. First of all, it is the same manufacturer we are talking about, so it's not Audi having one methodology vs Mercedes having another vs Toyota having a third. It is Honda vs HondAcura. Second, if they truly had more HP to wring from this engine, I would think they would report every last HP they could on the Acura side. In the segment it is competing in, you can never have enough HP. Which would lead one to assume that they are already reporting the highest HP they can.Manufacturers under rate their engines all the time. That's why a "280hp" 4cyl audi has a better performance than an engine from another mfg at "310hp"
Some manufacturers give SAE ideal hp, some give what they expect the worst case (added elevation, increased temp and humidity) and that leads to the under rating.
When they are decided how to label the hp in the car, they are using the competitive market to decide how they position it. If they think the car is underpowered in the market they rate it as high as they can to make it seem better. If they know it has better power than the competitors they will under rate it so that when customers drive the car they are impressed that it feels so much faster with the same or similar power.
The only thing that matters is back to back comparisons on same day same dyno, or 1/4mi with DA noted. Manufacturers just straight up lie about hp (under and over) all the time.