[No Spoilers] why do some of the songs have different versions on the OST than what is played on the show?

I know its not most of them but the big one for me is "a lannister always pays their debts". In the show when it plays at the end of the s3E7 the bear and the maiden fair, it sounds completely different with a loud brass section playing along. In the OST it is only strings and drums.

Submitted September 28, 2021 at 11:17PM by JacobBastian58 https://ift.tt/3AMaGZj