If the DPR of your display device is greater than 1, quest for asking yourself what is my screen resolution might have left you wondering why is the info from screen resolution test here different than the resolution listed in the official specifications of the device?
Answer is that applications, including your browser, use CSS pixels while manufacturer of your device gives display resolution info in physical pixels is a realization of the xundefined monitor.
Device Pixel Ratio(DPR) is a number given by device manufacturers and it’s used for HiDPI(High Dots Per Inch) or Retina(Apple’s trademark) displays, which are part of modern smartphones, tablets and even some laptops and monitors.
DPR is the ratio between physical(device) pixels and logical(CSS) pixels in either horizontal(width) or vertical(height) direction of a screen.
In other words, DPR is a number used for calculating CSS resolution of the screen. From DPR we can directly see how many actual physical hardware pixels make up one CSS pixel.
High pixel density devices were introduced to the consumer market in 2010 when Apple began shipping its products iPhone, iPad and iMac, equipped with xundefined displays.