Actually, a way to test this would be to use the current level penalty to ones advantage.
For example, take a low-level character and have it do a bunch of attacks (say, 100 or so) to a monster (where they only do 1 damage), then do one attack on a character that does more damage (Energy Bolt, or something that does over 100 damage… maybe even an auto-attack).
If the low-level character still has aggro, chances are that like Fuwaro said, threat may be calculated differently than from damage itself.
Edit: I’m not sure if there’s also a threat penalty due to level difference though… so now that I think about that possibility, a test would have to be done between players of equal levels and a monster with the same level in case there is threat penalty based on level difference.