Because you made a choice somewhere along the way that changed the dialogue options. I kind of think I may have done that in one of my plays - 3 of them - of this game. I remember killing the beast twice, but not the third time. I don't think any of my 3 playthroughs of this game were the same as I chose different actions and options each time.
As Jenny said, definitely a discussion for the Dark Side forum.
In my PC corner for the winter, gaming, knitting and reading.