Won't claim to be smart (hahaha!). But just eyeballing that, the difference appears to be subtle. Notice at 06Z, there is a western low center right along the Delmarva and another one to the east. The "eastern" one looks stronger (1000 mb), maybe the GFS keyed more on that at that time? At 12Z, the same thing...a "western" and "eastern" center. But in that case, the western center is stronger...in fact quite a bit stronger (992 mb), and the eastern low is farther out and weaker.