Apps use 2 streams:
Tethered: Standard stream quality when connected to WiFi.
Roaming: Mobile stream quality when using data.
Tethered apps on WiFi use the standard stream quality - that's anywhere between 16kbps to 320kbps. Roaming apps using mobile data switch to a lower stream quality at 64kbps or less.
The idea is to ensure listeners stay connected based on where they are. So less buffering, more feel good shows.