Q:

"It was Bob and Tom" – why not “They were”?

What is the rule of concordance/agreement in the following sentence? 

It was Magnus and Claudia

I have come across the type of agreement as in the example sentence above quite a few times. 
Could anyone explain why the instance goes as It was and not as They were and is the latter usage possible and if yes, in which case?

A:

I heard a knock and opened the door. It was Magnus and Claudia.

"It was" refers to the existential situation, the present fact.  This would also be idiomatic:

I heard a knock and opened the door. Standing there were Magnus and Claudia.

But this would not be idiomatic:

I heard a knock and opened the door. They were Magnus and Claudia. NO

This would be OK:

I heard a knock and opened the door. It was two people bundled up against the cold with scarves around their faces so that only their eyes were visible. They were Magnus and Claudia.

P.S. For the sixth-grader:

Knock knock!
  -- Who's there?
It's us.
  -- Who's 'us'?
  Magnus and Claudia.

A:

In a comment, you mentioned how you need to explain this to a sixth grader. With that in mind, I’ll offer this: 
In language, sometimes two are treated as one.
Here are some examples: 

I had a sandwich for lunch today. It was ham and cheese. 
My favorite concert was the one I attended in 1983. It was Hall and Oates. 

In those examples, the phrases “ham and cheese” and “Hall and Oates” are treated as a singular unit. The phrase “ham and cheese” could just as well be “ham on pumpernickel” and the concert band could have been “Metallica” or “the London Philharmonic Orchestra". Lexically, I’m merely describing the sandwich and the music group. 
However:

I had two sandwiches for lunch today. They were ham and cheese, and peanut butter and jelly. 
I have two favorite music groups; they are Hall & Oates and Simon & Garfunkel. 

This time, there are two sandwiches, and two music groups, so we need to switch from the singular was to the plural were. 
So, back to Magnus and Claudia: 

The phone rang. It was Magnus and Claudia. 

but: 

I invited two friends to the party; they are Magnus and Claudia. 

