Maybe it's all just semantics, I'm not sure.
Let's break the readings down into steps. It sounds like the 6% potential you have, is the potential of just the juice as it comes from the jug.
When you added the sugar, if it was properly diluted into some of the warmed juice, and then mixed thoroughly with the rest of the juice in the batch, and allowed to cool, should give an increase in the potential alcohol.
Likewise with adding the apple concentrate.
What were the SG numbers of the original specific gravity, plus any other gravity readings if you made any during the process.
I find it useful to take gravity readings of the juice before adding anything, then again after adding each component.
Also, when you get a chance, take a gravity reading of water at 60 degrees F, and see how close to zero your hydrometer reads.
If your hydrometer calibrates to zero properly, then take a sample of regular, un-tampered with, apple juice, and give us it's reading.
Pogo