Why not? From late April until mid-June he fit near or at .300 for the Cubs last season. I realize that is a small sample of games, but, that is almost 2 months full of a 25 year old first year player in the bigs. Not too many guys are able to pull that off. It wasn't until middle to late June that he began to dip down to about .250 where he pretty much plateaued.
As a minor league baseball player, Colvin batted around .300 much of the time too. His last season in Tennessee he hit around .300. I once heard a scout on radio where I live say that hitting .300 in AAA isn't like hitting .300 in MLB, it's actually more difficult due to less rest and very little scouting of the opponent's pitchers. He said if a guy can hit .285 or so, that's comparable to him hitting .300 in the bigs. He was speaking for AAA (as we have the Twins AAA here) but i'm assuming it's like that all across the board in the minors)
Point being, he's always been a good hitter, and he proved that to be true in the majors last year.