Electrical resistance

I bought a new zapper (it is my my third zapper). This zapper has adjustable voltage between 5-15V. On the first day I got the zapper - my husband used it on 9V. The zapper has detection for hands - if you do not hold the handholds - it stops working. After he finished I wanted to use the zapper. The zapper started to work on voltage of around 12V for me! My husband tried to measure my elect. resistance(with some elec.equipment, but do not ask me how, I am not technical type)and it was double of his resistance. I am wondering if there are more people out there for whom probably 9V zappers do no work. I am curious if people are born with higher elec. resistance or they can develop it by using zapper, rife machine, beck's devices, etc. every day.
BTW I killed some small Ascaris with my first zapping.
How does actually rife machine work (EMX+F165)? My rife machine helps me a lot, but so far I was not able to kill my huge ascaris, I have to get rid of it to stop producing new ascaris. thanks for any comment.