The rate of loss is not the important factor as much as charting overtime. Different bodies and deficit levels function different ways. Again, the constant focus on assuming your measurements are right in the first place is the downfall in this discussion in my opinion.
To write something like "from last week on a 4000 cal deficit... Booo!!" is putting to much faith of flimsy numbers. You are losing inches and charting downward overall. It is highly likely that you are storing some water, and you can refer to my follow up post today on my blog if desire for what you could do to test a whoosh out.
Get yo whoosh on
As for it being possible to accumulate mass in such a harsh deficit, well again we don't know what level of deficit it is to begin with. Next, if you are training in ways that are new to you or progressively pushing forward it is very possible to gain slight mass and at least, and more likely, possible to pull in extra glycogen.
I get the curiosity, and if you want to play around with water manipulations on smart levels, feel free. That being said, at this time with everything charting positive and the fact that you are losing inches and not much scale weight, that again points to the fact that your deficit may not be as large as you think it is.
It could be that age/conditioning/hormones are more of a factor for you and you could be off by 100-200 here or there. It could be because you are in good shape as well, not necessarily bad.