Google's 2017 Pixel 2 XL was one of the first phones to adopt LG Display's pOLEDs (6" 1440x2880) - and initial reviews of the display were rather dismal - as both reviewers and customers complained about bad color reproduction, graininess and problematic viewing angles. Many users also reported serious image retention issues.
In 2017, Google used Samsung's Super AMOLED displays in the smaller in 2017. This year, for the 2018 Google Pixel 3, Google chose SDC's 6.3" 1440x2960 Super AMOLED for the larger Pixel 3 XL - with proved to be a top-notch display. For the smaller Pixel 3, Google picked LG Display's 5.5" 1080x2160 - probably hoping that LGD managed to improve its displays - and most importantly its quality controls.