My testing seems to indicate that Attuned IOs are treated as their lowest possible level for conversions so I believe this behavior is working as designed. You see the same behavior when trying to convert an Air Burst as it is the only level 10 Ranged AoE set. It is an unfortunate side effect that these 2 sets only have 1 (uncommon) set in their category at this level so can only be converted by rarity.
I haven't tested it yet, but I suspect that this is always the case, it's just more obvious for these sets because they are the only sets at level 10 for their respective categories. For example, converting an Attuned Karma (10-30), will only ever get you a Kismet (and vice-versa), never a LOTG (25-50), despite them having some overlap. This is because it is treating the attuned Karma as a level 10 Karma in order to determine what is in the conversion pool. Essentially, you can never "move up" the level range tree, only stay the same or go down.
I suspect this was the best way to ensure Attuned IOs don't allow something a leveled IO couldn't do. Otherwise, for example, a Far Strike (10-25) could be converted to a Devastation (30-50) by going through an intermediary Maelstrom's Fury (20-35), whereas that could never occur with a leveled one since they have no overlapping levels.
Good to know when converting attuned IOs.