there is no interference if the slit is infinitesimal.
But in this regard infinitesimal is what's smaller than a wavelength (maybe half wavelength as I don't remember the exact solution).
But if it is larger than that you can say that it is composed of several infinitesimal slits next to each other with 0 distance between them. You do get interference.
I think you can understand the second part, if you look at the distance between the dark parts. They have the same width and their distances from the middle are the same
