Energizers also vary. Some deliver higher energy levels at lower voltages to an animal than other energizers. So the guidelines are just that - guidelines, but not scripture. So, when you read voltage, the total situation must be taken into account before concluding which is good versus not good. One needs to take several readings in order to observe a trend, and the readings are not always transferrable between energizers.
Animals react differently to an electrical shock.
Remember that a canine has bare pads on the ground and a bare, usually wet nose. Thus the lamb's resistance may be 1,000 ohms and the dogs only 250. That means the dog receives a much great percent of the delivered energy than the lamb. The same effect can be observed by humans touching the same fence in bare feet versus in shoes. The shoes increase the total resistance a great deal and thus decrease the actual joules delivered.