iOS screen recorder

Hi All

I have been playing with the screen recording feature on ios. I noticed during testing that if I recorded the screen with the youtube app and then played back you had sound of the video running. When I did this for a video through a webpage on Safari (need to record some online training) and then played it back, it had no sound.

Am I missing something here as seems odd it would record the sound from the YouTube app but not a video in safari