I’m not 100% sure, but I think it’s because of the difference in width and height. I’ve seen certain images that were too wide or too small show up weird because WordPress does its best not to over stretch or distort the image.
If you add the image directly as a background in your css then it shows up at the regular height….it’s only when the slider is being used it doesn’t show the full height for some reason. I also noticed that when I view the image directly it shows up at 237px height unless I click the image to zoom in, but when I click the image it shows up blurry.