2003 Busch (now Nationwide) body styles at Talladega and Daytona?

I know that back in 2003, Nascar brought fourth a new, larger chis to the Chevorlets, Fords, Dodges, and Pontiacs. These chis were used as well in the Busch series. But I noticed at the Talladega and Daytona races the Busch series used the 2002 chis. My question is, why?