[EricH]

Because they never do the exact same thing twice in a row.

Here's what I remember the hints pointing to:

2003: math formula, used for scoring
2004: some lines from "Stairway to Heaven", describing the stairs in the middle of the field
2005: riddle involving lots and lots of references to "three". Number of points per tetra and number of robots on an alliance.
2006: riddle shaped like the game piece. several elements of the game
2007: still from the game animation, showing the game object and some of the scoring structure.
2008: IR board (intended for use during hybrid); GPS coordinates (pointing to a spot indicating a race); one-line "riddle" that turned out to be the decryption password
2009: Picture of a moonfish (game theme); 4-line riddle (describing field and game pieces)