For some games (for example Heroes III) in "Bonus content included for FREE with the game:" there is "soundtrack", and in other (for example Sacrifice) in Bonus content there is "in-game soundtrack". What is the difference?

Some of the music comes from in-game recordings, some from official orchestrations. I guess.

Or one is the officially released soundtrack, while the other is a collection of all the tracks that are actually used in the game.

EDIT: Which may or may not be the same as what Foxhack was trying to say.