Here is some medical explanation for that:
a) The body realizes it is in starvation mode, and after a while, it says "Hey, this is too much, let's stop it for a while or use the resources more efficiently"
b) If you are exercising, you are building muscle mass, this weights, and adds up to your total body weight (naturally !
)
Now, if you (I do the same, so I don't blame you) weight yourself every week, that is not good, since this operation is based on long term results. My doctor tells me to weight myself every 3 months.. at least for the first year...