So let's look just at the throttle opening at WOT. Nothing more.

What limits the engines horsepower.... The displacement and the venturi opening.
If the day is cold and the air dense enough, the venturi opening will be adequate to pass all the gas the displacement will demand. And the venturi might be too large for those conditions, but it won't matter to the full power problem. The displacement will limit the amount of gas into the engine and we'll have to adjust the needle to give the right fuel amount for the right air amount for that venturi size. And we wind up with whatever the engine wants.

But on a hot day and a location with less dense air, the venturi can be what limits the engine. The available air that can get through the venturi won't necessarily be the amount that displacement can use. We still have to adjust the needle to get the right gas density. And if the opening is large enough for that hot day or that flying site elevation, the carb just might give the flyer fits on colder days at lower elevations. Why..... because then the airflow will be lots slower because the venturi size is too great and the engine won't get well mixed air/fuel. And we'll have fits setting the needle, and the lowspeed might also be off. So the carbs don't always suit all conditions. Or each of us's conditions.

Control line competitors noticed this and would carry a set of different size venturi to important contests. R/C engine designers decided to use the carb to vary the opening. But they're stuck with limitations. And they know that most modelers won't notice. So we see full horsepower when the throttle isn't completely open. Or the other way. Depending on how well balanced the carb design is to the engine's gas flow design and the engine's displacement.