Looking at 2:20, you can get a good look at all the spring connections. If you look at (we'll call it left is 1, right is 4) slot 1, it's barely holding on. And when you were testing slot 3 after putting the spring back on, it was barely staying in. I imagine that it will pop loose again down the road. The hooks are not curved enough for those holes to stay connected under tension. Also, as you mentioned about the connection of the spring to tab, that's a poor connection. I would of soldered all those springs to the tabs to make a 100% solid connection and would ensure the springs would 'not' pop back out down the road.
Also, I'm sure those springs have a decent amount of resistance in them. Personally, I'd solder in a wire directly from the tab to the negative lead. That way I know it's gonna transfer the proper amount of voltage/current.
Now we know why they can have spurious readings, especially with resistance, when wiggling the battery or putting more tension on the tab. It's a weak design at that spot. The weakest link.
