3500 calories always equals one pound. I think (based on what I've read on several threads here) there are numerous things that make a difference. One of the things, at any stage in weight loss, is water in the body. Perhaps you have maintained a 3500 cal deficit over a week but you only see .5 pound on the scale. You still may have lost 1 pound of fat, but perhaps you are retaining some extra fluid so the NET loss is lower, numbers-wise.
The other thing is knowing for certain what your deficit is if your body is not being fueled properly to handle the activities you perform. Now, someone will correct me if I'm wrong here, but I think the idea is that the same activity that burns 400 calories when your body is properly fueled (as in, it HAS those calories to burn) may burn fewer calories if your body is desperately trying to conserve energy in order to simply function. So if you are starving yourself, you may not be burning the cals you think you are, therefore your deficit is less.
Or that's the general idea. And I'm sure there are numerous other factors to consider.
If you are significantly under eating, then your body steals resources from your muscles, bones, organs, etc. in order to function. There's a limit to these resources. If you're on a plateau, your body could be rearranging muscles and fat, just settling around, getting comfortable with the routine, etc. That's why some people recommend zig zagging in order to break a plateau. The body is tremendously adaptable and does get pretty efficient at conserving energy when there is a need to.
You might have noticed that when you first started working out it took a great deal of energy and sweat and as you got better at it it did not require as much. In order to get the same caloric expenditure you would have to add intensity or duration to your workout.