Yes, the "Mill" discussion looks to be totally irrelevant. [1] and [2] seem to back up my claim that, at least in modern times, it's purely a "just-below pricing" psychological trick and has nothing to do with the Mill unit.

$4.999 looks a lot smaller than $5.00 to everyday people and it makes the gas company more money than $4.99. That's all there is to it.

1: https://www.cbsnews.com/minnesota/news/why-do-gas-prices-alw...

2. https://www.cnn.com/2022/06/14/energy/why-gas-prices-fractio...