Another theory, if something not happening does not count as change, than maybe it doesnt go down recorded path every time, and instead, goes down one path or the other, but when it goes down the other (AKA we dont see it go through the recorded path/slit) and it forms a frequency pattern. The other half of the time it goes through the recorded path and forms a line pattern (more to that side).

side note: if you used 2 separate cameras to record each path individually, the result as I understand it would be the same, the change made in the world is still there, regardless if its on 2 sep recording devices, or even screens/pixels etc... so dont let that fool u xD