Copper and Aluminum are normally used in electrolysis, and Carbon and Magnesium are as well, and Carbon/Magnesium are also on opposite ends, and they do not produce voltage from just being connected because there is not enough ambient moisture. I can take a picture if you want.
I just understand the basics and I am still learning. I was able to recreate this, but only by putting tons of water on the board in the same area. On looking at the photo though, there was water on the board, so I think in this case you are right. I missed that.Do you understand the Galvanic scale, or do you not?
Graphite is anhydrous, making it highly absorbent. That (of course) keeps the contact quite dry.
But go ahead and think you have discovered a magical voltage source if you want.
You've made it obvious that you don't want an explanation, although galvanic corrosion at contact points entirely explains why your voltage is dropping.