I've encountered this quite a few times now with different Youtube videos. Not just with this video.

I thought I'd see what key he was playing in with the tune at 07:50. The audio is indicating key of F#. That couldn't be right so I slowed it down to see where his fingers were landing. Sure enough....key of F (at 08:05 frame).

Now the odd thing is Jalousie at 06:30 he's doing in C and the pitch sounds like C!

Then....at the frame at 09:30 you can see he's playing in the Key of C but it's coming out as key of D.

Why does this happen with a lot of Youtube videos?

He is using the transpose button!

Or he has saved a key change in the registration.
John C

Good thought,but it is not consistent throughout.

Or they changed the key so YouTube wouldn't be able to track the songs played.

That doesn't fool YouTube. I made a video from a Karaoke file for my friends to practice to, and posted it privately to my own YouTube account. I used my BK and transposed it two half-steps down. YouTube still immediately flagged it as Copyrighted material.

They didn't ask me to take it down (because if it's private, i guess I'm within the doctrine of Fair Use.) But they did say it couldn't be monetized. Bottom line, changing the key, at least for a realistic rendition of a song, doesn't fool YouTube's copyright detection.