About 10 years ago I tried an experiment where I used the same speed settings on a film cameras as I had used on a digital camera. It didn't work - the film was exposed very differently to digital (I don't remember if it was under/over.) The ISO number that you get when you take a picture with your DSLR is not the same as the ISO number used for film. Try it for yourself.
That seems like a difficult experiment to conduct. There would be too many variables to use negatives and actually quite a few variables using transparencies. Did you develop the film yourself? What were the controls used to assure that temperatures, etc., were precise. If you sent the film to a third party to be developed, you lose all control over the process.
How were the camera's calibrated. Did you verify that the mechanical shutter of the film camera was correct? Film cameras are notorious for the shutter speeds being off. Was it the same lens on both cameras?
How did you compare the two images? Was it two prints? Transparencies?
This conflicts with the whole idea of having ISO anyway (the "s" being for standardization). If it were different between film and digital, light meters, etc. wouldn't work properly.
I've heard of people routinely using their DSLR to meter for film cameras. I tried it once myself and it seemed to work. Of course, this is just an anecdote, and Unfocused made the important observation: the "S" in ISO is for "Standard." I.e., there's a group out there that tests this stuff in a standardized way.