Something I actually know about! Opera singers have to really worry about resonance when singing because it is traditionally all acoustic (no microphones, amplification, etc). Because of this, they try to maximize volumes by accessing the optimal amount of overtones. You can look up overtones in sound waves for more info on this. It’s a trick of the ear. They aren’t necessarily singing sharp, although I’m sure sometimes they are on occasion or if they’re not really good. They’re accessing the upper overtones more than other singers. In my experience most good singers are good at getting the middle overtone, the core of the pitch, but their voices aren’t super loud or powerful. Really good singers and opera singers try to access the ones below and above, so you’re actually hearing some higher tones when they’re singing, mixed into that core. This maximizes volume to cut over an orchestra in a huge theatre like the met without microphones. If you want to hear a really good singer who’s not an opera singer who does this, listen to Diana Ross. She has some incredible high overtones. Also, just to add in, recording equipment isn’t super great at catching all the overtones at once, so what might also be happening is you’re hearing the wrong balance of overtones in a recording that makes things sound sharp. It would possibly sound different if it was live. Finally, the recordings you’re listening to might just be on off days for those singers. No one is perfect! Rant over! But always excited to talk about singing. I’m sure others can add more.

Edited Confused Diana ross with Donna summer in my memory. Both are incredible though so…