GoForSmoke:
The electrons flow from - to +. In doing so they create "holes" that measure as flowing from + to -.When you measure +V it is always relative to something. But grounds should all be the same.
If I put a capacitor between a 5V pullup and ground, the + side will lose electrons while the atoms left behind still have all their protons and the ground side of the cap will fill up with extra electrons exactly to match. If you read it, only read the + side and it will make it a tiny bit less +. Arduino can read that pin at around 100KHz, it adds up quick.
Thanks.