Why is the transmission or distribution voltages always multiple of 11e.g 33000,6600,11000,440..etc.

Why is the transmission or distribution voltages always multiple of 11

e.g 33000,6600,11000,440..etc.

The above  values are at the sending end and include an allowance of 10 % for line drop , giving voltage at the receiving end of 33,000 6,000 10,000 ,400volts etc