No, it's always "per unit that you use". Units that you buy and not use are wasted, so that's would be the wrong way to compare.
Dressing up an obvious idea with fancy-sounding math formulas and pages of text is exactly how you get trapped by someone good at selling stuff.
You're right, but I think you're less likely to buy extra at per-unit pricing (since you can always buy just-in-time), so you're less likely to run into a situation where you overpurchased at per-unit pricing. (Edited to note that.)
I'm sure we've all had an experience at least once where we've bought something in a larger container from the grocery store thinking we were getting a deal, only to find that we didn't get through it as fast as we thought we would, it started spoiling, and we got to scrape our savings into the trash can.
And many of us have had an experience where someone higher up in the company has insisted that we use a tool that isn't very good "because we've already paid for it", so it's clearly needed advice, even if it's not groundbreaking. Everyone sometimes needs "obvious" conclusions pointed out to them, I think. It's a quirk of our mental processing as humans.