please explain fragmentation to me, the dummy in "Fragmentation for Dummies".
I am not really qualified to explain it properly but I will try. I don't consider myself any more knowledgeable about the subject than an average person. The best way to describe resolution in my mind is to say that it is the amount of "lines" of pixels (rows and columns) the display has. For instance, looking at text on the iPad Mini Gen. 1 (1024x768 resolution) you are looking at the number "1" on a webpage (zoomed out all the way - so the font is small). There might be, let's say,
12 rows of pixels making up the number. Whereas on the iPad Mini retina (2048x1536 resolution) would have
24 pixels for that same number, zoomed out just as much. That simply means that it is sharper looking than the lower resolution screen, across every aspect of the display - because there are more lines and pixels.
PPI is pixels per inch. This is how dense the pixel count of the screen is. The higher the PPI the sharper the screen looks as well. I believe the reason for this is because the pixels are smaller, in order to cover the same resolution, so because they are smaller (since the screen is smaller) but still need to cover the same resolution, each area of the screen (per inch - as it's called) is more dense/covered in more pixels, which in turn makes it look sharper.
Generally speaking, the bigger the screen, the smaller the PPI. If you were to have a 1080p screen, on a 18" Computer monitor, it would only be around ~140 PPI (complete estimate) whereas on phones like the HTC One and Galaxy S 4, they are 5" screens with a 1080p display. That means they are over 400 PPI, which means they are incredibly sharp.
Oh whoops I just realized your question was about fragmentation

. Well fragmentation in this topic relates only to resolution, PPI has nothing to do with it. In order to make apps, they need to be made for a certain resolution (how much real estate the screen has) - because you don't want to have an app that isn't optimized for a certain resolution, and have the edge of the app cut off! Apple has been smart about keeping fragmentation to a minimum. They use Pixel doubling when they can, which basically means it keeps the same "screen real estate" while doubling the resolution. This keeps apps from needing to be optimized to multiple devices. They did this with the mini also. The iPad mini retina is exactly double the resolution of the iPad mini, therefore no apps need to be optimized. They just need to be tweaked easily to double the resolution (which happened when the iPad 3 came out - as the full-sized iPad and Mini retina now have the same resolution).
Apple iOS lineup only has 4 resolutions. (3 if you ignore the iPhone 4/4S). The iPhone 5/5S, the iPad 2/Mini, and the iPad Mini Retina/Air.
IMO, 326 PPI will no longer be the maximum. The "iPad Pro" will need to be greater than the 264 PPI of the Air, because the Pro is a higher product line. Also, the iPhone 6 will need to be higher than them all, as the distance from face is:
1. iPhone 6 ("Retina+", > 326 PPI)
2. iPad mini with Retina Display (326 PPI)
3. iPad Pro 9.7" (264 PPI > "Retina" > 326 PPI
4. iPad Air 9.7" (264 PPI)
5. iPad Pro 11.5" (264 PPI > "Retina")
I don't think you understand how PPI works. Inherently, the bigger the screen, the lower the PPI. The 13" Retina MBP has a higher PPI than the 15", because the 15" is a bigger display. there is no "Pro" products. If apple releases a 12" iPad, it will have a higher resolution than the iPad Air, but likely still a lower PPI, because it is gaining 2" in size.