This is a test of the camera's noise levels. The higher the signal to noise ratio, the greater the difference in strength between the real image data and random background noise, so the 'cleaner' the image will look. The higher the signal to noise ratio, the better.
Signal to noise ratio charts
JPEG signal to noise ratio analysis: The Nikon D7200's JPEGs are clearly noisier than those of the Canon D760 and the Pentax K-3 II, which are so close together they can't really be separated.
Raw (converted to TIFF) signal to noise ratio analysis: The pattern is repeated in the raw tests, though here the Pentax starts to show a slight advantage over the Canon as the ISO values increase, and Pixel Shift Resolution mode widens the gap.