Park effect is a small factor. Many other events conspire to keep a prominent player down. A strong reliever in late and close situations, intentional walks, great fielding, etc.

This statistical event happens to everyone---all the time. Players under achieve, and this is especially frustrating when you use them in the middle of your order everyday. But look at your numbers, I am pretty sure that some guys have exceeded their stats. No one ever talks about this, just guys who don't live up to their current stats.