Why does converting a .ts stream to .mp4 result in a longer duration

Question

Using ffmpeg to convert a .ts file to .mp4 format sometimes results in the converted video's duration being longer than its original.

What would be causing this to happen?

I've only found answers online that point towards timestamps and different primary and sub channels being one of the reasons.

I use the following ffmpeg command to convert the files:

ffmpeg -i fileName.ts -map 0 -c copy outputName.mp4