My personal theory is that those with ADHD have compressed dynamic range of the dopamine behavior/planning feedback loop. Neurotypicals have a smoother control loop with more resolution. ADHD brains have less DA throughput, but the brain still tries to "crank up the gain" to compensate. The result is a response curve that's closer to a step function. Every molehill of a task feels like a mountain because the activation energy is high. Conversely, once you get in the groove, doing anything else feels like trash because the current hyperfocus is so rewarding.

Haven't found a ton of research to back this headcannon yet but there's evidence that the D1/D2 ratio is extremely important.