Ground and neutral are not the same. There should be no current flow in the ground line, so its potential, relative to true earth (usually in the UK, referenced to the substation) is close to zero. The neutral line carries the return current, so its potential relative to true earth.

Also supply GND is not necessarily system ground - your centre tap example shows that. The centre tap is merely a 0V reference. In many cases though, it important to reference this 0V reference to GND NOT, REPEAT NOT to NEUTRAL.

Steve