The latitude and longitude, height above sea level, atmospheric stuff like particles, moisture, and everything else in the air affect how the sunlight gets filtered. It’s the same as the temperature being different in neighbouring cities or even neighbourhoods but generally on a larger scale. Most things have an effect, even stuff like what kinds of plants there are in different countries can have an effect. Imagine a building with lots of rooms in a line with windows. Everything from curtains, every piece of furniture, where in the building it is and how hot and dry the air is inside each room will affect the light inside that room. Same with cities, countries and continents. There are so many things that can affect it, so inevitably all things together will end up giving different results.