Does this mean market prices are private, not public?
My guess (guesses are all we are going to get until a dev steps in and says how it actually is ^^) is that prices are driven by demand and supply. If a station has a high demand and no supply of commodity X then they will pay a lot for commodity X, let us call that value Y. If someone with 400 units of cargo comes along they would get 400*Y.
If on the other hand you come in with 10 units of caro and run repeated runs (40 to reach the 400 cargo capacity of the first example), you would get 10*Y + 10*0.9Y + 10*0.8Y.. 10*(1-0.1n)Y.
My guess is the fix is supposed to account for that when you sell large quantities. The effect could probably get pretty extreme if Y does not decrease linear with supply, but rather exponentially (which makes sense, prices escaleate quicker when you get very desperate for a product).
/Merf - Guesswork!