OK, I could have made it very plain by saying how I think about it, maybe using some hand gestures.
So, I make a fist with my RH, and say "this our 'take-away' number, that we're subtracting, called the subtrahend, but that's not important."
Now, with my index finger in the LH I point to the top of my fist and say, "I'm call this our 'index number,' the very top number of our subtrahend. So, it's 13 in our example, right?"
Now, I say, "When we take away our whole number to be subtracted, we're taking all of that number away."
I lower my RH and leave my index finger pointing where it was in mid-air.
"But now we've lost our index, and we need that back, so we're just going to add it back in while leaving the lower numbers gone."
I think that's the best way I could explain it in person.
I don't think that would have been patronizing or "mansplaining" or whatever, just a clear visualization.
Plus, I get to use the word "subtrahend," which I was happy to remember correctly. Really not a word you get to use every day.
ETA Oh, I should have pointed out, so to speak, to her, that the managers when assigning routes to fill up this "spots" on the floor did it wrong and we in fact comfortably have room for 39 vehicles in that section of the warehouse, not 38. So the spreadsheet warriors got it wrong, as well.