Measuring negatives from some Nikon (F2/F3/F4/FM2 ... don't know which) the width is indeed oversize at 36.12 mm.
I take it you are scanning mounted slides and not the film itself. With the slide mount doing some cropping, and the scanner probably doing some more to remove the fuzz around the slide mount aperture, I'd say some change in aspect ratio should be expected. If a constant amount of crop is made in the horizontal and vertical dimensions the aspect ratio will change to a higher value.