The description above appears to be using 1.10 or earlier - is that right?
Its resolved / different (depends on your perspective) in 2.0 with the new edit functionality.
In 2.0 when you attempt to open a sent survey you get two choices 'Edit and resend' or 'Copy to a new survey'.
In the first case - 'Edit and resend' - if you do what is described above, you will see the survey gets moved to Drafts, and indeed if you delete the draft, you no long have a copy of it on your device. The original is still in your feature service, but you cant edit it from the Sent box any more. If you do need to edit sent surveys you should configure the Inbox.
In the second case - 'Copy to a new survey' - a copy is made in Drafts but the original sent survey remains in the sent box.