It’s hard to imagine noise on your line only affecting one portion of the frequency range. So in other words if you reduced noise that’s being measured above 20khz why wouldn’t you expect that same filtering to affect the entire range even in areas that aren’t being measured by the meter?
This problem is kind of a Venn diagram. A conditioner which starts at 10 kHz will hopefully also clean up 100 kHz. A conditioner which starts working at 100 kHz won't necessarily clean up 10 kHz.
Of course, coupling can occur later, and there will be a frequency high enough that it stops working at too.
So, you have a meter which measures RFI, which shows reduced noise, and the power conditioner happened to reduce noise in the audible spectrum too. That's luck.