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Here's a few notes of the top of my sleepy head:
Have you tried different camera profiles in ACR? I don't have a Pentax so I don't know if the range of profiles available for it are as extensive as they are for Canon and Nikon. Have you tried creating your calibrating your camera to create your on custom profile? This is very much recommended. Then you can create custom variants from that.
ACR has to render the raw bits into a presentable image as does the Pentax software. While they use similar techniques, there are differences. And any of the custom 'scene' settings that you may set in the camera are probably ignored by ACR as they have nothing to do with the raw bits.
If you were to use a third program to render your raw file, you'd also get different results.
ACR works in ProPhoto, not Adobe-RGB. You only get Adobe-RGB when you save the raw file out to a different file format (eg tiff or psd).
Then end result of all of this is that you'll likely never be able to get an exact match bit-level between images rendered by the two programs. As you gain control over ACR you can, however, get results as good or better than you are able to with the Pentax software. It just requires some work up front to learn the tools available. Jeff Schewe's book is the best single resource for learning about Camera Raw processing.
Thanks! I started to deal with thoughts like this during the day, and now I understand better how it works.