The image is 266px x 342px so the area (.jacketb) to show the upper half should be 171px.
Now that height of .jacketb happen to be 174px, it should be 171px to fir the image.
Also the background-size could better be the actual image’s size than the given percentage that always suffer from rounding errors. I think that is the reason for the off position.
When you’re using percentage for dimensions, you always risk the browser’s zoom level to add extra rounding errors as well.
When using percentage it’s good to make sure the rounding error is accounted for. In this case it was not even when the browser zoom was other than 100%.