Isn't this simply because of the spread in effectiveness between characters of the same class at different levels?
An unoptimized 5th level cleric is going to be almost as effective as an optimized 5th level cleric simply due to the lack of options. Contrast that with a 15th level cleric and how much of a spread there is between unoptimized and optimized.
An unoptimized 5th level cleric is going to be almost as effective as an optimized 5th level cleric simply due to the lack of options. Contrast that with a 15th level cleric and how much of a spread there is between unoptimized and optimized.