The mysteries behind engine balancing?

Balancing means different things to different people. In terms of my stuff, it measn the lower end is internally balanced to a tolerance of less than a gram. That means the small ends of the rods, pistons, rings, pins, and locks all weight exactly the same. The big ends of the rods all weight the same. And the bobweight is precisely offset by the weighting of the counterweights. (and yes - it's balanced just like a tire once the bobweights are attached to the crank. It's spun in a saddle with sensors to note the location and amount of imbalance.) The factory took the path of 69 Cuda... "close enough will get us by".
In terms of the OP, the shop years ago included the flywheel, disc, and pressure plate on the crank when it was fitted with the bobweights during the balancing work - and they added weight there. That's one of the issues with external balancing - which is what that old shop did. Internal puts the counterweight within the block's main saddles which makes it less stressful on the block and parts. External makes it smooth but adds stress to the block and crank. In your case it also voids the warrany on any parts they might have to modify...lol