there is no interference if the slit is infinitesimal.
But in this regard infinitesimal is whats smaller then a wavelength (maybe half wave length as i don't remember the exact solution).
But if it is larger then 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 distance from the middle are the same