Seth Masket graphs it:
I had figured earlier that these differences were a result of the fact that Obama just had an early head start while Romney was still slugging it out in the primaries, but by this late date, I'm not so sure. It seems more like the campaigns have different philosophies when it comes to deploying campaign resources, with the Romney folks believing they can win in the air and the Obama folks believing this will be won on the ground.
Update: Masket tweaked his graph because he can no longer "confirm that either campaign has a presence in Indiana." The newer version is now above.