Differences due to cable can still be legitimate physical events considering:
1) Different cables can be made of different materials. I'm sure you will find that many people swear by the difference between silver and copper audio cables.
2) Electrical interference from outside sources. We live in an age where some sort of wave (which is a signal in itself) is always constantly passing through. It's possible that even if you have not changed the system, an interference from outside sources may hit the system somewhere and cause the changes. That's why we have shielding for our electrical components.
I'm not sure how you conducted your tests, but there are surely variables that can cause physical differences to occur beyond your control (like the interference).
I actually went and did a lot of reading and research after perceiving the differences between these 2 headphones myself, and realized that there were way too many variables to list. Still, I maintain that there is a difference between them, and it's not as big a difference between, say, the M50 versus the DT770, but it's surely as big as the difference between the DT880 250 Ohm and the DT880 600 Ohm.