Is there a “true” 1/2 inch size?

Hey y’all! Sorry if this has been asked before or if this is a dumb ass question LOL. So 1/2 inch has always been my goal size and I’m currently there at 12.5 mm. I knew that 12 mm was also considered 1/2 inch, but I also just learned that 13 mm is too? I probably won’t go up past 12.5 mm because I keep having dreams of my earlobes splitting or melting off my head (I have no idea why because I baby the hell out of my ears), but I’m just curious as of why three sizes are considered to be 1/2 inch and if one of them is the like… the truest 1/2 inch? I hope that makes sense and I’m probably overthinking this LOL.