I’ve noticed this as well, but I was having trouble articulating it. I believe my situation should align with yours.
- Card is created with audio (sound.mp3) on Anki Desktop.
- Sync on Desktop.
- Sync on AnkiMobile.
- On desktop, I open the file in MP3Gain to raise the volume of the mp3 without re-encoding.
- Sync on Desktop.
- Sync on AnkiMobile.
Result: Anki Desktop has a louder sound.mp3 than AnkiMobile because the change didn’t come over.
In this example, the name and file size of sound.mp3 wouldn’t change, but its content (volume) would.