Some .mp4 record as .ts without converting...why?

[Barak-Har]

I sometimes download video files online with .mp4 suffixes that turn out to have zero information attached to them aside from their filename/size, but which play perfectly. To get them to list their length in Windows file manager I copy each on VLC as they play, in the normal method of VLC recording, but the new file, although now displaying length, always ends up with a .ts suffix instead of an .mp4, despite the fact that I haven't asked to convert them. Does anyone know why this happens?

[Lotesdelere]

The record button is recording streams in their native formats, no transcoding is performed.
So it's either a TS file with an MP4 extension, or it contains streams that VLC can't mux to MP4 so it's muxing them to TS.