Google updated the way Gmail suggests contacts to you. Up to this point, if you emailed someone five times, Gmail automatically move them into “My Contacts”. Now, Gmail no longer automatically add contacts, instead, you can go to Suggested Contacts, select the contacts you'd like and move them into My Contacts. All of your contacts -- whether they're in My Contacts or Suggested Contacts -- will continue to show in auto-complete as you're composing messages.
As part of this change, previously auto-added contacts are moved back into Suggested Contacts. Only contacts that you've edited, imported or added to a group will remain in My Contacts for a clean slate and a better point for syncing contacts with mobile devices (for example with Android).