Found Good explanation to explain what Retina Display is ;)
sorry for my english :D
You need to understand what retina display is.
retina is the definition of how a human eye cannot differentiate each pixel.
the display becomes a retina when you hold the device to a certain distance where you cannot see a pixel at all.
For instance, the iPhone is made to be held one your hand, so the ppi is larger, since you are holding it closer than a mac or iPad. The reason they have a lower ppi is because you hold and see your device further away than an iPhone, so it does not need as much!
source: youtube comment