You might miss to include a friend in a mail but GMail is going to suggest you whom all you could include based on what you have in the To/CC list. The suggestions appear in a hyperlink and you can click to add. The logic is recent and common. So if you add email@example.com then it would suggest you b.xyz.com, firstname.lastname@example.org and other. Now if you select email@example.com now it would suggest the names common to firstname.lastname@example.org and email@example.com which might not include firstname.lastname@example.org.