Why do some ORCID iDs end with an X?

ORCID identifiers that end in X are valid and correct. 

ORCID identifiers are all randomly assigned and cannot be changed. The last character in an ORCID identifier is a checksum. It ranges in values from “0” – “10,” with X representing the value “10.” The checksum ensures the other 15 digits of your identifier form a coherent iD.

You can learn more by reading about the structure of an ORCID identifier.