The two terms are frequently treated as translations of each other. They describe different objects with different purposes, and using one where the other is intended produces predictable results.
A masala is assembled for a dish
A masala is a mixture of spices proportioned for a particular preparation, and a household may keep several, each with a different balance and a different intended use.
The composition varies by region, by community and by family, so two cooks describing the same named blend may be describing quite different things.
Because it is dish-specific, its proportions assume that certain other ingredients are present, and it will taste unbalanced in a dish it was not designed for.
Curry powder was built for standardisation
Curry powder emerged as a single blend intended to reproduce a general impression of Indian seasoning without requiring a cook to hold a spice cabinet.
Its composition is fixed by the manufacturer and usually leans on turmeric, coriander and fenugreek, which gives it a recognisable and consistent character.
That consistency is the point, and it is also the limitation, since the same profile is applied to every dish it seasons regardless of what the dish contains.
They are added at different moments
Many masalas, particularly the aromatic finishing blends, are stirred in near the end so that their volatile compounds survive to the table.
Curry powder is generally cooked in early, because its ground turmeric and coriander are raw-tasting until heated and need time in fat to lose that edge.
Swapping them without adjusting the timing gives either a harsh raw note or a blend whose aromatics have been boiled away before serving.
The base flavour is built separately in Indian cooking
An Indian dish typically develops its foundation from aromatics, souring agents and long cooking, with the spice blend contributing one layer of a larger structure.
Curry powder is designed to supply most of the flavour on its own, which suits a cooking style where it is the only seasoning added.
This is why a dish made with curry powder tastes uniform across the plate while one built with layered masalas tastes different in each mouthful.
Neither is a substitute for the other
Curry powder used as a finishing masala tastes flat, because it lacks the concentration of aromatic spices those blends rely on.
A finishing masala used as a general seasoning tastes overwhelming and slightly medicinal, since its aromatics were never meant to carry a whole dish.
Understanding which job a blend was made for is more useful than judging either as more authentic, since both were designed around real constraints.