Thank you for the inquiry into the NGC Registry.
Mint error coins are unique, many times one-of-a-kind and are valued completely differently than regular issue coinage. They are typically more valuable than a similar “regular” coin and are also harder to assign a value because their worth can be very subjective, simply based upon a collector’s desire to own a mint error. Here is NGC’s description of Mint Error coins: https://www.ngccoin.com/news/article/7765/learn-grading-mint-errors-part-1/
Competitive NGC Registry sets are composed of coins where many collectors can obtain and own the same type of coin. These coins all receive similar registry scores. As such, mint error coins, due to their unique nature, are not assembled with regular coinage in competitive NGC Registry sets.