He mentioned that it can happen for a few different reasons...
1. Sometimes its because the mom can't actually eat as much because lo is sitting too high (this has definitely not been the case with me as I find I'm eating either more or normally)
2. Less fluid retention i.e. loss of water weight (Which I also don't think happened to me as my feet & ankles are still swollen at the end of the day)
or
3. LO grows at a really rapid rate of about 1 lb per week at the end of pregnancy & may just be burning more than mommy can consume. (If I had to guess I'd say this is what's happening to me even though visually doesn't appear to me that my bump is any bigger)
I'm sure there are other reasons, but those were the ones he mentioned as being the most common.