From Patch Notes e1.4.1
We changed price colouring to work according to world averages instead of default prices of items. If an item is 30% or more above the world average it is coloured red. If it is 20% or more below the item's world average price it is coloured green.
If I had to guess, I'd say that this change inadvertently broke goods prices globally. The price coloring system is tied into the global market, and somehow price changes for a good anywhere are causing the price change for that good to happen everywhere. Essentially causing the normalization of prices globally over time. AI caravans might be contributing also.
Just a guess.
Edit:
After more testing, I'm pretty sure that the AI caravans are what's causing goods pricing to normalize (likely thanks to the e1.4.1 update above). At the moment, trade rumor prices are completely unreliable, often wildly so. For example, a trade rumor says that you can sell beer at Myzea for 90. When you get to Myzea a few minutes later, the buy price is down to 40 and they have 200+ units in stock.
My theory is that the AI caravans are utilizing the now globalized list of goods prices to buy and sell faster than any player could. That, I suspect, along with the undocumented changes to goods prices, is causing the global normalization.