Ionut noted that Google is rolling out an updated “Contact Manger” for Gmail, that’ll no longer automatically add to the list of Gmail contacts all the people you've sent an email. Instead, the updated version has a separate section for the people you've emailed called “suggested contacts”.
“My Contacts is a place to import, store and view all of the contact information that's important to you. You can also create your own groups of contacts to easily email many people at once. We add people you've emailed to Suggested Contacts. You can move contacts from Suggested Contacts into My Contacts at any time.”
There's an option to automatically move suggested contacts into My Contacts if you frequently email them, but the previous behavior doesn't seem to be an option.