What determines the order of "recent" texts when you type +1 in the "To:" window?

I am trying to determine if these are the most RECENT texts or most FREQUENT texts. Dialing "+1" in the "To:" field of imessage, will bring up a "recent" list- even if the text thread and/or the contact have been deleted. If the "recent" has been deleted from your phone, it will appear in a gray font with an "i" to the right of the number. If you click on the "i", you are given the option to delete the number from the "recents" list- but are NOT given the same option if the contact remains in your phone.