what you are describing is a perfect symmetrical, isometric cube, which has no vanishing point and thus is impossible to actually exist. What you are trying to show is that the the cube is not perfectly set up like this:

In a correct cube with a vanishing point. those green and pink lines you drew, should not be symmetrical, they should go towards each other.
For example this cube:

You see I drew all the same lines you did. and these didn't math either.
Only if I wanted to draw a perfectly symmetric and isometric cube, these lines would match up.