Why are 2003 - 2005 US nickels different from the others?

The nickel's reverse side was changed to honor the bicentennial of the Lewis and Clark expedition, which was authorized by the Jefferson Administration.

However only 2004 and 2005 nickels have different designs, not 2003 nickels. Political wrangling over which images would be used prevented changing the 2003 coins, and forced the Mint to use the then-current picture of Jefferson on the front of 2004 coins rather than one of the newer portraits used the following years.