First of all, park factors, like all baseball stats are subject to random variation. Why should we be any more surprised by park factor fluctuation than by fluctuation in player batting averages?
Second, ballpark features change all the time, even when dimensions don't change. Vary the amount ot foul territory, put up a new scoreboard, change an entranceway and you can change the park factor.
Third, everybody seems to forget that a key variable changes ever year. WEATHER. Temperature, humidity, winds, pressure all vary and have an impact on the game.
Fourth, remember that park factors are always relative to the rest of the parks. Your park may stay the same, but if every other team is building a park that boosts offense, then your park may move from being a hitting park (with hitting stats above the mean) to a pitching park (with hitting stats below the mean).
Fifth, there's not one park factor for a stadium. A stadium affects different measures of offense differently. GABP is a great example, because it's like Camden Yards.
Both are good HR parks.
Neither is a good offensive park, because they allow fewer doubles, triples, and singles.
Good HR park does not = good offensive park
Gor more on GABP and park factors,,,,
http://www.hardballtimes.com/main/artic ... factoring/
http://www.fantasyinfocentral.com/theho ... lparks.php
http://www.sportfanatics.net/Baseball/F ... istics.htm
http://philliesblog.blogspot.com/2004/1 ... isdom.html
Here's a park effect crib sheet based on those published in the "Bill James Handbook 2005":
AL Best Offensive Parks: The Ballpark in Arlington, Fenway Park, SkyDome.
AL Best Pitching Parks: Safeco, Tropicana, Jacobs Fields, Yankee Stadium.
NL Best Hitting Parks: Coors Field, Bank One Ballpark, Wrigley Field, Citizen's Bank Park.
NL Best Pitching Parks: PETCO Park, Great American Ball Park, Dodger Stadium.