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 firstname.lastname@example.org then it would suggest you b.xyz.com, email@example.com and other. Now if you select firstname.lastname@example.org now it would suggest the names common to email@example.com and firstname.lastname@example.org which might not include email@example.com.