What does being from Cal have to do with anything?
It basically means he probably went his entire college career without having to against a really tough defense. Here are some backs from the Pac-10 who have had some success in the NFL in recent years: Corey Dillon (Washington), Steven Jackson (Oregon State), Reggie Bush (USC), Maurice Jones-Drew (UCLA). Bush did well his rookie season but if you remember things looked pretty rocky for the first half of the season, and at times he seemed lost. And he was the most highly-touted running back in recent memory.
Sjax was in a timeshare his first year, but he looked lost at times too, and struggled his first full year as a starter just barely breaking 1,000 yards.
Dillon's rookie year was so long ago it's kind of hard to compare to today, but he did well despite not starting, averaged 4.8.
Then you have MJD who absolutely killed his rookie season.
So it might not mean anything, it might mean that a back's running style determines how quickly they adapt, I don't know.
But here are some blue-chippers from the SEC, and their rookie seasons:
Addai (LSU), considered an average back in the SEC, topped 1,000 yards in his rookie season despite being in a time share, and scored 7 times
SA (Bama) averaged almost 5 ypc his rookie season despite getting only 64 attempts. The next season he took over and topped 1,300 yards and had 14 Tds
Ronnie Brown (Aaaaaaaaaauuuuuubuuuurrrrrrrrrrrnnnnnnnnn!) almost topped 1,000 playing behind Ricky Williams and managed a respectable 4.4 ypc, but scored only 3 times. His following year I probably don't need to point out was somewhat of a disappointment.
Caddy (Aaaaaaaaaauuuuuubuuuurrrrrrrrrrrnnnnnnnnn!) topped 1100 despite missing two games with injury and being severely limited in others. In fairness, he was overused in the first few games, averaging something nuts like 27 cpg as a rook. Not smart.
Travis Henry (Tennessee) absolutely sucked his rookie season
Jamal Lewis (Tennessee) topped 1300 and 300 carries his rookie season
Deuce McAl (Ole Miss) only got 16 attempts his rook season but topped 13000 as a soph, and certainly with that few rook carries his soph year was really his first in the league
Fred Taylor (Florida) broke 1200 and had 14 Tds his rookie year.
There could be a pattern and it could be totally randar, but it would make sense that backs who play in the toughest defensive conference in college football (if anyone wants to argue this, just look up the DROY and DROY runner up from last year) would be more NFL-ready than backs that come from a much weaker conference on defense (Pac-10).
Now, there are no good SEC backs in this draft with the possible exception of Irons who has no shot at starting this year or next, so this doesn't matter this year, but last year, if you were looking for an impact rookie of the big 4 (Bush, D-Will, Maroney, and Addai) Addai was the best bet and he wasn't even considered a top talent in the conference. Of course this is not a causal relationship and certainly being in one of the more explosive offenses in the NFL didn't hurt, but NO was not far behind Indy in offense last year and it didn't get Bush past 1,000 rushing, and Bush was practically deemed the second coming of Gale Sayers in the offseason.
All this to say that I could easily see a subpar year from Lynch, and a great second year. Since Caddy should be primed for a bounce back with O-line additions and QB stability/improvement, I think he's the better bet.