I don't understand why there's two sets of tracks provided (DTS-MA and LPCM) that offer the same resolution........
Because only LPCM is mandatory for BD players to be capable of decoding in 5.1
Neither DTS-HD MAS or Dolby True HD are supported for lossless 5.1 as a mandatory codec.
DTS is mandatory for Core Audio only, and Dolby True HD for stereo. Early Profile 1 players may well ot be capable of outputting lossless from DTS-HD MAS or Dolby True HD, and will instead output DTS Core Audio and Dolby Digital in these cases, both whilst still lighting the "True HD" or the "DTS-HD MAS" lights on the receiver as these lossy streams are embedded into the bitstream for just these situations.
DTS-HD MAS is usually picked over Dolby True HD as the support is widest, and the hidden core streams sound better too, plus with DTS you need one file - .dtshd - to cover every possible situation whereas with Dolby True HD you have to author with individual .MLP and .ac3 streams, keeping them in sync etc. Much more hassle.
In summary, the only way to guarantee lossless 5.1 at 24/96 on a BD disc is to use LPCM, as all players must be capable of outputting this.
If you use just DTS-HD MAS there will be cases where the actual output is not lossless, and the same applies to True HD.
Hope this clears it up!!