What's odd is if you set a trap's danger to Setback, say the party has two characters affected by it. One 4th level, one 5th level. The 4th level character takes 1d10 damage. The 5th level character takes 2d10 damage. From the same trap.(I'd never play it that way as DM, but it's something odd I noticed.)
I believe the intent is that you work out a level for the trap based on the level of the party. That would be average level, usually
