Color me surprised that you're back. Tell me, how does the size of a screen cause pixels to magically shrink?Because they set out to make a Mini, the ppi is determined, CAUSALLY, by the size of the screen.
No really, because you've just solved the entire industry's problems. If only we make the screens smaller, the pixels will play along and make themselves smaller, too! Just like that!
The size of pixels is one of the only two things you can control. "ppi" is a measure of dot pitch is a measure of pixel size. That's all.The ppi in this case is completely out of their control.
You can calculate it empirically if you are holding a finished display in your hand and know its exact size and its native resolution. You can also calculate it with a microscope. You can also calculate it electrically if you know a few parameters about the LCD controller. There are plenty of ways to calculate the figure, but there's only one cause: the size of an individual pixel.
It's really not hard to understand if you stop and use your brain for just one moment.