Reading this LWX AFD -
The 07/12z GFS
phases upper level energy ejecting eastward from Colorado with
northern stream energy diving through the Northern Rockies, while at
the same time kicks the northeastern US system east of Nova Scotia
by Sunday morning. This allows for increased amplification of the
incoming system and higher probabilities for precipitation. The
07/00z ECMWF solution on the other hand keeps the upper level energy
unphased, and the northeastern US system atop Maine/Nova Scotia
through Sunday, leading to a more suppressed weaker solution.
The thing is that if you compare the two solutions at 96 hours side by side you dont really see much different in the placement of the upper level features.