Audio on BD - WHY does it happen ?

[Ray0002]

I been upgrading most of my DVD movies to Bluray

When I say most ... I am talking about over 100+ and counting.

Each time a old movie gets released on Blu Ray I catch myself scanning the back of the box to determine the Encode Transfer and Audio options.

I pride myself to have built a considerably nice Home Theater system

Upgrading my hardware as new technology becomes available.

Currently using : LG 65 LW6500 3D LED , Dennon AVR-790 7.1 Polk Surround
and one monster 20' JBL sub woofer.

So why is it that some BD movies come with less than HD sound ?

As a newb i will list in order the best audio to least desired on BD as I understand it.

Please if your an audiophile , feel free to correct me if I am wrong.
I need a clear understanding of HD AUDIO to help me make better buying decisions.

PS Thanks in advance.

1. LPCM aka PCM : Linear pulse-code modulation
"uncompressed PCM" Typically used by SONY

2. DTS-HD Master Audio : uncompromised audio : carries one encode

3. Dolby® TrueHD : With support for up to 7.1 100 percent lossless audio
carries two encodes.

Now more and more often I am seeing movies with less than the standard audio options listed above.

Aeon Flux for example : Audio options: English: Dolby Digital 5.1
:English: DTS 5.1
These were the same encodes used on the regular DVD version.

Young Guns : Audio Options : English: Dolby Digital 5.1 EX
: English: DTS-HD 5.1 \
The DVD version from the Special Edition
English (Dolby Digital 2.0 Stereo), English (DTS 5.1)

I want to know why and how these BD versions are considered : HD
I also want to know why on this Blu-ray Forum these audio formats are not being addressed in the Audio Section:

From the review on Aeon Flux
"Accompanying the excellent Dolby Digital track is an English 5.1 Dts track"

I may be new on here but this is just not satisfactory to me.
If Bluray Movies boast to have Best Picture and Sound in pure HD then
how come they slap the original DVD encode on BD ?

My Denon AVR 790 has a LED blue indicator light which pops on when a HD audio source is being played.

When it doesn't come on when playing a BD movie , I cant help but think I have been taken advantage of.

Any comments are welcome.