Why is going on with AUDynamicsProcessor?

I've been doing audio production for 25+ years and development for 15 years but AUDynamicsProcessor has me baffled.


I want to add a hard compessor to crunch down peaks. So I set threshold to -3dB and headroom to 2dB. Attack time is set to 1ms. Source audio is peaking at +2.5dB


But the settings above have absolutely zero effect. Peaks are still above 0dB.


I have to actually set the threshold to -13dB to get peaks below 0dB. Why?