Voltage is the potential difference between two points in the same circuit/system, so you weren't measuring a fluctuating 'voltage' as such, just witnessing the meter's attempts to interpret the electrical potential of two isolated systems floating relative to each other. By connecting the meter (which has a very high but finite impedance) you were creating a connection and affecting the result.
Think of it as trying to measure the length of a stick by comparing one end of it with the other end of another stick, when you have no idea how far away the other stick is. Except in this case, the act of measurement will tend to reduce the distance between the sticks by an amount that is also unknown. Basically, the results are meaningless. You need to fix the two sticks together.
OK, rubbish analogy
Submarine communication systems engineer and amateur robot enthusiast.