Sourdoughs are made from a combination of yeast and lactic bacteria. These are living microorganisms, and when paired with flour and water, they trigger a fermentation process.
As this process occurs, environmental factors such as humidity or air quality affect the bacteria and the fermentation, giving your sourdough unique flavors.
At the same time, natural bacteria in the air can interact with your fermenting sourdough. These other bacteria can also change the fermentation process slightly.
This is why sourdoughs have unique tastes depending on where you ferment them. For example, sourdough in Central Europe tends to taste like grains such as wheat and barley.
You can also narrow things down even more and look at how sourdough's taste changes within individual countries. For example, Belgian bakers tend to make sweeter-tasting sourdough.