My interpretation goes a bit like this, and forgive me for the metaphor here..
You are in a room with a stereo with an electronic face that lights up when powered on (which it is), and the volume is muted.
I associate presence or absence of evil with stereo power, and aura with stereo volume. Just because it isn't making noise doesn't mean you are unable to discern whether it's on or off.
Now, even though this goes a little beyond the scope of the initial question, but to expound upon my example, say the volume is set at max before it was muted, and the 3rd round (or pally ability) occurring effectively unmutes stereo... the overwhelming volume is jarring, much like an overwhelming aura is when the relative HD's are far enough apart.