I've been quite confused about this one thing while comparing video stats in PotPlayer.

I used 2 different software to download a (free) video online. The other one was an mp4 and the other mkv.
Both were 1080p and around the same size but because Windows had trouble with listing the bitrates and resolution of the mkv file via the files properties, I played them side-by-side in PotPlayer and right-clicked 'Playback/System Info'.

Ever since then I've been wondering why in the 'File Info' tab of the mkv video it said under "Audio # 1" - 'Delay relative to video : -80 ms'.

I never noticed any delay in the audio or video, so is it something that happens often or is it just me?

With the mp4 video it doesn't even have the option of 'Delay relative to video' so I assume PotPlayer doesn't recognize it, yet I don't notice any difference either way and I prefer mkv as an container and would like to use the software that downloads as mkv instead of mp4.
I know that both the mkv and mp4 downloaders use ffmpeg I'm pretty sure and other stuff that I don't know anything about because I didn't build them.

The mkv one only needs the link of the site where the video is hosted (similar to yt-dl/yt-dlp) whereas the mp4 one works by pressing 'F12' on the site and copying the 'playlist.m3u8?token=' -link from the Network tab (I don't know if that makes any difference).

But what does this 'delay relative to video' mean, can some PotPlayer-user or anyone smarter than myself explain it and how does something like this happen?