When a map is zoomed out, the colour that is assigned to the pixel does not take all blocks in that location into consideration. Usually, it just assigns the colour of the most common block. This can cause anomalies such as bridges disappearing over water. See screenshot for an example.

What should happen instead is that all blocks in that location contribute equally to the colour of the pixel and the assigned colour of the pixel is the average of all blocks in that location. This will make features show up more consistently and make zoomed out maps look less blotchy.