CRTs had limits on how quickly the beam could scan across a line (and return to the other side of the screen for the next line), which imposed a tradeoff between the number of lines per frame and the number of frames per second. Within a line, the number of pixels per line was often limited either by the speed of the graphics card's DAC or the analog bandwidth of the VGA cable. But sometimes it wasn't, and you could take a monitor originally intended for something like 1280x1024 and get it to display 1920x1080 with acceptable sharpness, after adjusting the picture height to compensate for the changed aspect ratio.