The resolution is higher because the display is larger and more "lines" can fit on it, the PPI is still the same though and that's what affects what things look like on the screen.Well I use my iPad on a table or on my lap. My phone is held up in my line of sight and is much closer to my eyes and face. I don't need to hold my iPad close because it has a a much larger display.
However the resolution on the ipad is much higher than the resolution of the Xr.