How does this SQL code work?

I was studying SQL and found the following question:

What is the result of the following query:

SELECT SUM (V2.VALOR - V.VALOR)
FROM VIAGEM AS V INNER JOIN VIAGEM AS V2
ON (V.VALOR = V2.DISTANCIA / 100)

And the table:

    Viagem
Valor Distancia
  3       200
  4       100
  2       500
  12      400
  4       300
  3       200
  12      100

The answer is 20, but, I did not quite understand the operation. I put the script to run, I did some tests, but I did not quite understand the operation. My reasoning was as follows:

For each value of V.Valor I will compare with each of the other values of distances / 100.

For example:

For the first value of my table V, I will compare with each (v2.distance / 100) to see if they are equal, and so I am putting together the resulting tables in order to make the sum difference. Is that it?