Products in Mercatorio takes up space. And they reside at specific geographical location. Movement of goods is affected by their size, and this is an important mechanic for shaping the economical landscape. So while jewellery or medicine might come close to having a global price, firewood most definitely doesn't, as cheap supply in one area might not economically be transported to satisfy high demand at far away places.

Source and More information: Space, storage and transport

Author: indieDB.com