The shadow is diffuse, rather than sharp. The ESA explains this by noting, "If you were standing on the surface with Rosetta high above you, there would be no place in the shadow where the entire Sun would be blocked from view, which explains why there is no fully dark core to the shadow."
The image was taken during a close flyby of the comet on February 14, but the ESA just now brought it to the public's attention. Rosetta — which was launched back in 2004 and sent on a mission to approach and study Comet 67P — was at a distance of about 3.7 miles from the comet's surface at the time.
What's so intriguing about the shadow image is that it's something familiar happening in an alien place, 317 million miles away. We're all used to seeing our shadows here on Earth. Rosetta casting a shadow on a comet puts its epic space adventure into a more human perspective.