Pcm Vs bitstream

[Eidolon]

Although I have a surround sound system, for various reasons I won't run it if I'm watching something in mono or stereo.
I either use the TV speakers, or a soundbar depending on how fussed I am about the audio of what I'm watching.

I've noticed an oddity when using Bitstream audio to the TV. Now I'm aware that bitstream is meant for audio receivers, but it's a bit of a nuisance switching between audio output formats, and it generally works on my TV.

However, when I have a machine set to bitstream out, the TV (or soundbar via the TV) is prone to making volume changes for some reason. It mainly seems to effect things on dvd with stereo soundtracks, such as old TV shows. The big problem for me is that my default machine, a Philips, only outputs in bitstream via hdmi, so I have no choice on it with that, and it's total pot luck on what it will effect.

I've been trying to find a way to stop it from happening, switching different things such as Night Mode, down sampling etc, but nothing seems to stop it from happening.
If I have the audio set to pcm out, on the other machines, then this problem doesn't occur.

Does anyone understand why this happens in the first place? I was hoping that there was maybe there was something I overlooked that could be suggested to remedy this annoying problem.

Thanks