Why are subtitles in films or tv shows italicized some of the time?

[MJW93]

I noticed this especially in HBO's hit series Game of Thrones, I was just wondering y this occurred because when i watch this show I feel like the subtitles become italicized for no apparent reason most of the time?

[Nikse]

It could be because the person who talks is not visible, but it could also be other stuff...