It's pretty typical since most of the ratings are based on a few highly rated prospects that usually start the season at High-A/AA/. Over the course of a few years many of those arrive at the majors and surpass eligibility time or flame out. As far as an inability to maintain year to year much of that has to do with team philosophy and how teams change once they usally benefit from having a good farm (like the Cubs after '03). I think comparing the Cubs and Sox, it isn't that Cubs got better as much as the Sox having some interesting drafts and no splashes as far as international FAs, which is why they overhauled their amatuer scouting dept. There are some teams besides Anaheim (LAD, ATL, CLE, TB, NYM, etc)that consistently rank higher than the Cubs, the Cubs had their stretch early this decade but once Prior, Cruz, Z, Hill, and Choi weren't prospects anymore, they fell off quickly.