AVR receiver's 'red/danger' level

I have a Yamaha RX-A1080 that I've used for quite a while and still love - was fine with my similar old one but needed 4k support. Anyway, I didn't really use the app to control it until I moved and the remote hasn't turned up. It will, but this is fine for now.

When using the app I've noticed that if I put it louder than -20db, the volume becomes red like it's warning me. I don't get anything outside of the app and have run it louder than this enough times. 20 seems like a nice round number too, so why that?

I'm powering a 5.1 system with Polk ES-20 in front, ES-10 in back, and whatever the center speaker model that goes along with that is. Then a powered sub but that doesn't require much output from the AVR.

While most things like streaming, blu-ray, video games tend to be best around -30, or more like -35 to -25, I recently got a turntable. I'm running it in 'straight' mode which as far as I understand tells the receiver to leave it alone aside from directing lower frequencies to the sub. I find records need to be at at least -25, and sometimes -15 - unless I turn on some digital processing, enabling any DSP, even just to use the rears and center for 'all channel stereo', brings it back up to the level that my other devices (all digital) run around -30.

Why's it seem like I should be concerned about something at -20? After years and no problem, I'm not too concerned, but curious.