Plaster colors darker than white are never completely even, i.e. there is always some mottling. Your color variation is about what I would expect, though the pattern of the variation is more unsightly than normal.
Yours looks a lot like mine--especially the roundish splotches. I've heard that a no-drain acid wash might help, but it reduces the life of the plaster. I just live with it. It's only really noticeable in low light. When the sun is out it looks fine. And I don't believe you have calcium scale. This is the condition that Onbalance has written about.
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