Or lets take your example:
As I already mentioned, this only means anything for specific situations where Supes is up against someone, and that someone is shown to match them pretty much exactly. Though even then, you get things like PIS and SMvsFL or what not, but disregarding all of that, it's still a remarkably specific situation that doesn't come up often enough to make a real difference.
Your statement is only true if character A and character B literally have the exact same strength.
I'm probably doing a poor job explaining this.