What is the significance of 75mV in available 100A current shunt resistors?
There's no special significance, shunts are available in different current ranges and different full scale output voltages.  Some applications may not want the 100mV drop in voltage on a 100mV full scale shunt, others may want the higher signal.  Some are designed to work with particular meters and therefore have whatever scaling factor that meter requires.  If you need a 100mV shunt try Newark Electronics: www.newark.com