Singular, plural or both?

[Mori]

John and Bob are at the door. John pushes intercom button and then says: "it's John and Bob". There are two of them, though. So why not "they/these are John and Bob" ?

 

[Anglika]

Welcome to the forums.

"It is something" is a common usage to indicate something.

It is nine o'clock
It is not your affair
It is the best thing that ever happened.