They had to wait for additional characters to make a good guess as to whether it was Z or SE. So they thought that having your fingers already positioned close to both outcomes would allow you to handle that delay better. Maybe it allowed you to catch back up faster, or made it easier to handle both of those threads in your brain and nerves? Perhaps whatever letters came after Z/SE were unlikely to also be Z/SE, so it made it more likely your right hand would be typing the next character?