Ask HN: Subtitles with Accumulated Time Error

I wanted to add subtitles to an old movie that I have but I only found the Blu-ray version of the subtitles. I thought it would be okay, I would only need to shift all the subtitles a specific amount of time to resync them. But to my surprise, as long as the video progressed, the subtitles were more and more delayed. I can't grasp why this is happening because to my understanding, the time between phrases should be the same, no matter the format of the video. If more frames are added, making the scene longer, the audio would be desynchronized so I don't understand this behaviour. Can anyone enlighten me about what's happening here?