There’s up to 10% more muscle activity in the decline version versus the flat version. More muscle activity means more muscle fiber recruitment. That means more fibers are stimulated to grow. This might be in part because the triceps are stretched more. The muscle being stretched the most is often the muscle recruited the most.
The decline version is also more effective at isolating the triceps because the shoulders are taken out if the equation – they tend to compensate in flat variations. And the decline position places the triceps under tension for a greater range of motion and more optimal loading.