Adding Signers by name look up not working for all names

When adding signers I can type their name or even part of it and the email will appear making it easy to add them while others I must type part or all the email address. Why is that? Where are my signers name and email stored if anywhere or where is that look up feature pulling from so I can associate a name with an email? It is so much easier to just type a name (or part of it) and have the email appear.