Part of your problem with Rodriguez RBI total is due to the fact that over the first 25 games Rodriguez AVG, OBP, SLG is less than it should be. This can probably be chalked up to small sample size and will tend to correct itself over the course of the 160 game season.

The other primary difference to consider when looking at RBI totals is who is hitting in front of him. I don't have the research in front of me to know who hit in front of Alex Rodriguez all last year; however, I would venture to guess whomever it was got on base more than the combination of a pitcher (NL park), Luis Vizcaino, and Austin Kearns.

Less baserunners = less RBI opportunities = less RBI.