Lots of things may happen when music is committed to recording, not least of which is that the players were not tuned to a perfect 440 or whatever to begin with.
There’s no such thing as being “off key,” only “off pitch” which refers to an instrument or vocal not being in pitch with whatever frequency the music is tuned to. There’s no law that says we have to be at 440. One may prefer something different (likely a little lower…there’s a whole thing with 432 Hz)
Why is so much music recorded out of key?
As a guitarist, I like to sometimes listen to music and figure out a solo or sometimes play along with a song. My guitar is in perfect tune but a lot of the time the music is off and in between 2 keys. Like a semi tone off key. This really never happens when listening to a live recording, only studio recordings. I assume all the musicians tune up during a session and of course piano is already tuned to a standard. So has anyone else noticed this and can explain why. Don’t try to blame it on my turntable as it happens with digital streams and cd releases as well. Been noticing this for years. What gives??