If what Bill Hunt said on a Digital Bits (and follow-up Facebook post) is correct, Universal makes new transfers and does their mastering at 1080p; other studios make their masters at higher resolutions (2K or higher), and downconvert that to Blu-ray's 1080p. A theory has been proposed that suggests that while Blu-ray is 1080p, a higher quality source than that will yield better results than doing both the transfer and the Blu-ray encode at 1080p, if that makes any sense. Universal has been suspect of late, not that they do completely terrible work, but that their transfer quality (as opposed to encoding) may not be as high as other studios, which may explain some of the discrepancy with that.
The other thing is that American Graffiti was shot in 2-perf Techniscope, which would naturally produce a very grainy image; someone in the Universal mastering department, possibly even someone from Lucasfilm, might have decided that they didn't like the grainy look, and attempted to produce a release that was stripped of that grain; in other words, it might have been a (in this person's opinion, wrongheaded) intentional mastering decision.
I have yet to see the disc so I can't offer any further judgment than that. I'm sure it will look better than any previous home video release - the question is, is it accurate to the original film, or was a deliberate choice made to change the look to a certain degree?