I am trying to understand this reading
Explanation of the 1.71V you measure across pin 2 and 4 on the right side / secondary side:  

The outer pins (pin 1 and 4) on the secondary side are not being used/connected. They are just pins sitting there (and being just when you buy the 18/2/16). The resistance of the potting material isnt infinity.  

If it would be 115 MOhm and your multimeter's resistance would about 10MOhm, you will read a voltage of 21 V * 10 MOhm / (10 MOhm+115 MOhm) = 1.7 V