Recently noticed the number of items is lower than the owners of an NFT collection.

I didn’t pay attention to this issue before. I checked some other NFT collection and the Emojis of Cheers UP Official is also like this.

Usually, the items shows the total quantity of an NFT collection. To figure it out, I read the contract of these NFTs, and the NFTs that have this display issue are implemented ERC1155 contract. In comparison, ERC1155 does not have totalSupply() function as ERC721 does, which is how the number of items obtained by OpenSea. OpenZeppelin provides ERC1155Supply.sol extension for calculatations of totalSupply field. There is an article published by ChainSecurity exploiting a fixed vulnerbility of this extension : TotalSupply Inconsistency in ERC1155 NFT Tokens.