Because visitors to those sites include corporate and government users -- the majority of windows installations. Corporate and government computers don't have S

Because visitors to those sites include corporate and government users -- the majority of windows installations. Corporate and government computers don't have Steam installed. Those website stats also include home users, people with Steam, etc. It's a way bigger sample across all sectors, including the sector that Steam measures. However, Steam stats only measure windows installations of people who play Steam games (i.e. not corporate and certainly not government computers, and probably missing a large chunk of home installs). That is a much less representative sample of windows installations.