I can't explain the difference; but I have been the recipient of numerous jolts from "gun show" stun guns. I never took the time to check the voltage, who made it etc.
It originally started out in college. A friend had bought one for his girlfriend but was concerned with whether it was effective. Running around a farm growing up, I had done the ol' zap your buddy with a cattle prod thing and been on the receiving end as well - so after a few beers I decided to let them use me as a test dummy.
First test was just a zap on the leg - a little pain; but not much - mostly caused the thigh to spasm and was otherwise ineffective.
Second time, we tried the arm... some loss of the use of that arm but not much pain and everything else worked fine.
At this point, I felt confident enough to let them try it under the "ideal" circumstances described in the instructions - my arms lifted with the stun gun applied under the armpit to the torso. This particular method was painful; but absolutely ineffective - all muscles worked just fine. I got hit for about 15 seconds before I became irritated with the pain and grabbed it out of his hand.
Finally the brain being operated with tiny electrical signals, I began wondering if that might be a more effective zone. I later zapped myself right on the noggin... this was the most effective of any of the tests. No pain at all; but it was like someone was rapidly flicking the lights off and on and I dropped the stun gun immediately.
In general, I don't consider stun guns of any kind of serious obstacle and have yet to meet one that convinced me otherwise.... now Tasers are a totally different matter.
The older model of Taser was more like the stun gun I mentioned above on steroids. The current production models are really potent. I've seen several guys who I know can take a stun gun hit get stopped cold by a Taser. Of course, they still have the same problem as stun guns - as soon as the charge stops, the only thing stopping the person from resuming their activity is their psychological desire to avoid another zap.