The reason the game and the OST have different master is a couple of things, one: the mixes for the game are acting as BGM, so they need to fit in the background and give appropriate frequency space to the sound design. Helps the sound cut through better (It is not lost on me that the sound does not actually cut through all that well in places currently, rigorously balancing that as we speak). The game mixes also loop, in most cases, seamlessly, but there are a few tracks that start and stop into silence, must like their album version counterparts.
The OST album mixes are bigger, have no sound effects to compete with, and are more suited than the game mixes for dedicated listening, i.e. proper album mixes.We throw the Background out of Background music at this stage and are left with full blown, nothing but, music.
The OST album mixes are slightly different in form in some cases as well, some to extent, some to shorten (I thought Bounty Hunters wasn't much of a listening track so it got shortened for the album release, for example), and in many cases, proper intros and finales to each track. I'm fond of fading out steadily on a good feeling groove, so that does come into play here and there, just a personal preference for some, and others get a real exclamation point ending, just depends on how we felt about each track.