The company explains that the "slight shifts in color and hue" when viewing the screen off-angle (read: not straight on) are perfectly normal. It also says OLEDs exhibit slight visual changes with long-term use, such as showing remnants of a high-contrast image displayed on the screen for extended periods of time even when it's already showing another image.
Those two are also the most common issues Pixel 2 XL owners have with their Android Oreo devices. By pre-empting potential complaints, Apple is most likely trying to avoid facing a similar debacle. In Google's case, though, some customers' complaints might be warranted, since they reportedly got burn-in as soon as a week after their purchase.
Despite the warning, Apple assures customers that their pricey new phones aren't going to have less-than-perfect displays anytime soon. The company says it "engineered the Super Retina display to be the best in the industry in reducing the effects of OLED "burn-in." And, as AppleInsider notes, iPhone X uses OLED made by Samsung. The Korean conglomerate also manufactures OLED screens for Pixel 2, which doesn't suffer from the same issues as its bigger sibling.