I've always thought that the weight listed on a weight stack (putting the pin at 100lbs) meant that you were lifting 100lbs after all factors were taken into account. After reading this answer, I am uncertain about the meaning of the weight on the weight stack.
I have also recently switched gyms and went from 90 to 40 on rope pulldown, which I attributed to the conversion between weight units (It matches which could be a coincidence). Could it have simply been the different brand of machine that meant the weight is actually different due to the setup of the machine?