When someone mints on NFT on OpenSea using their own wallet as the identity of the creator, why does Null Address always appear in the NFT's transaction history as the creator instead? and then the second event in the history says Null Address transfers the NFT to the creator
Because the standard says so.
/// @dev This emits when ownership of any NFT changes by any mechanism. /// This event emits when NFTs are created (`from` == 0) and destroyed /// (`to` == 0). Exception: during contract creation, any number of NFTs /// may be created and assigned without emitting Transfer. At the time of /// any transfer, the approved address for that NFT (if any) is reset to none. event Transfer(address indexed _from, address indexed _to, uint256 indexed _tokenId);