The point is, PPI is much less relevant on OS X than on iOS. To
create Retina displays, Apple doesn’t have to build displays that
are exactly 2X current displays, they just have to build displays
that work well with OS X when running @2X. For example, the
current 27-inch iMac is 2560 by 1440 pixels, which translates to
109ppi. Doubling that to 5120 by 2880 pixels is not strictly
necessary. Such a screen might be incredibly difficult to
manufacture, and therefore incredibly expensive. Instead, Apple
could build a 3840 by 2400 pixel 27-inch screen that presented
itself as a pixel doubled 1920 by 1200 pixel display. That’s
effectively an 84ppi screen @1X and 168ppi screen @2X.
The key thing to keep in mind is that on iOS, the precise physical size of screen elements matters. A button rendered on an iPhone 4 uses four times the pixels as the same button rendered on an iPhone 3GS — but they’re the exact same size. That’s not a concern with the Mac, because the Mac is not a touchscreen device — and I don’t think it ever will be.