The margin math assumes every bed masks equally; the coverage scope checks whether this one actually does. While playing, it measures the bed against the voices across the speech band (200 Hz–8 kHz, weighted toward the consonants) and calls the result even, thin or exposed.
The advice holds the weakest stretch of the session, not the prettiest moment: a bed with quiet gaps is judged by its gaps. If it says exposed at a suggested margin, the bed has a hole where speech lives: pick a fuller bed or raise the music.