Thank you for the hint. Your quote didn't really help me understand anything, but there is a better quote on Wikipedia:
"Errors sometimes occur due to the technology used to measure the height of terrain; for example, tall buildings in Adelaide, Australia, cause one part of the city to be rendered as a small mountain, when it is in fact flat. Also, in Downtown Los Angeles, there is a large hill in the approximate location of Bunker Hill, where many of the skyscrapers are located. The height of the Eiffel Tower creates a similar effect in the rendering of Paris. Also, prior to the release of version 5.0 in February 2009, elevations below sea level were presented as sea level, for example: Salton City, California; Death Valley; and the Dead Sea were all listed as 0 m when Salton City is −38 m, Death Valley −86 m, and the Dead Sea −420 m.
Where no 3 arc second digital elevation data was available, the three dimensional images covering some areas of high relief are not at all accurate, but most mountain areas are now well mapped. The underlying digital elevation model has been placed 3 arc seconds too far north and up to 3 arc seconds too far west. This means that some steep mountain ridges incorrectly appear to have shadows extending over onto their south facing sides. Some high resolution images have also been misplaced: an example is the image covering Annapurna, which is misplaced by about 12 arc seconds. Elevation data was recently[when?] updated to 10-meter (1/3-arc-second) resolution for much of the United States from the previous 30-meter (1-arc-second) resolution."
The problem is obviously resolution. If resolution in some areas is 3 arc-seconds (90 meter) or worse then it's obvious that a 1 arc-second (30 m) wide creek simply disappears and that elevation changes in this range get averaged out.