sorry I don't quite understand what this is
how comes the solid red line is not a double slit diffraction pattern?
because thats what it appears me. There are three fringes from that solid red line occupying the space inside a wide central fringe (from the blue line)
I didn't notice this until you've point it out here. Yeah that is impossible, the distance between the two slit centre must to be greater than the slit width. It makes no sense.