Different OST in dubbed version

I wonder if any other users have noticed that the music in the English dubbed version is similar but,definitely,different than the original score in the French version.Furthermore,the nostalgic French song that the female singer sings accompanied by piano in the scene where Delon enters a piano bar is different.I noticed that when I finally got myself a French widescreen version.It seemed strange because I was so identified with the dubbed version score,especially in scenes like the opening title sequence or in a classic scene where the heroes have become successful and they happily come down the neighbourhood's stairs accompanied by a lot of people(somehow I can not even recall the movie without remembering this picture).
I can't explain the reasons for this alteration of the music score,but both versions seemed to work.Of course,it was necessary for the viewer to see the original and that didn't seem possible until the DVD version came out.