It has nothing to do with this model. There is no HDMI audio without sending video too... the audio is embedded. Now they don't have to send the same video signal to the audio only output but they do have to send video. And based on what type of audio it is certain resolutions (higher) in order to keep up.
A search or two will provide the gory details...
Since HDMI audio is embedded in HDMI video, and since HDMI video has a built in re-sync on every raster line, the amount of clock drift possible in clocking the audio out of the input signal is inherently limited to what can happen in the short amount of time that passes transmitting a single raster line of video.