Based on my knowledge of electricity, which is greater than my knowledge of anatomy & medicine, I think it would be more accurate to call it electrical resistance, not impedance.
Resistance is what impedes the flow of electricity or electrostatic charges. A higher resistance reduces the flow. A galvanometer sends a small current thru the test object and measures how much gets thru.
If you hold one probe in your left hand and one in your right, and the galvanometer measures the flow, the reading is affected by moisture, how good the contact is between the probes and your hand (squeeze tightly, more surface area, better contact, less resistance) and the body’s natural resistance due to skin and organs. Sweat more, less resistance. Room temperature and humidity are also factors; body temperature may be.
I doubt if your pulse or blood pressure would have much effect on the reading. And I know of nothing in medical science that uses such a reading for legitimate diagnosis (but I could be wrong, see previous disclaimer).
Why do some people think the position of the meter’s needle means something other than current flow? For the same reason some people think tea leaves in the bottom of a cup have fortunetelling value, or tarot cards can reveal your personality. Quacks will always be with us as long as there are fools eager to part with their money.