ARC/eARC is about the TV sending audio from HDMI devices to another HDMI port where your receiver/soundbar is connected.

HDMI audio passthrough is a feature that allows apps to send any raw encoded audio signal "as is" via the HDMI port (eg: Dolby, DTS, etc) to your receiver that will do the decoding.

The ATV does not support DTS or TrueHD, hence to output audio with those codecs, it needs to be decoded and converted on the fly to a supported format like PCM. Unfortunately this produces all sort of issues (sync, volume, etc).