The LCD's pixel response is slower than the refresh rate of the display, which presents as smearing when pixels need to transition from distant states (e.g. full white to full black or vice versa). The reason this doesn't happen on the iPhone is because the response times of pixels in the OLED displays they use is much faster. It's also why I don't think pro-motion is as big of a deal on the ipad / macbooks as it is on the iPhone.