Originally posted by grahame I believe you.
I guess aperture size on lenses are not 100% accurate. The levers are just fixed on the ring by screws, and workers adjusted them during assembly. that is why we see a little difference in exposure among lenses. That little difference might count.
Also, optical formulas contribute to the bokeh and the transition from in focus to blurry area. That might contribute to the slight difference in DOF at same aperture setting on the aperture ring.
Just my guess.
Not sure whether it is minor accuracy differences in aperture or more probably your second point, that the optical formula impacts the bokeh, and the out of focus area, so that the apparent separation of foreground and background is more pronounced.
Years ago I did a comparison of a few 135 mm lenses and found that an early preset lens produced more three dimensional images due to the much different optical formula.
As norm suggests you can measure the depth of field and will see it is the same, but it is the bokeh that might be making the FAJ 18-35 seem to have more DOF