Well, strictly speaking, it's not the usual Mercator projection, but the "Google Mercator" or "Spherical Mercator" projection, EPSG 3857 or EPSG 900913.
This is a Mercator-like projection that assumes the Earth is a sphere instead of an ellipsoid. Many online mapping systems use this projection because it is a much simpler calculation than than a more correct Mercator projection would require.
