Wow! It sounds like you are eating a very healthy, well balanced mix of food. If you are grazing through the day too, that is excellent for you metabolism!
I haven't been eating nearly as healthy as I should. In fact, I pretty much give in to whatever cravings I have. At 34 weeks, I had only put on 7kgs. I haven't really exercised either as I find it too painful to walk long distances (and I am lazy! ).
Everyone is different and pregnancy affects everyone differently. Some women stick to a healthy diet and exercise yet still put on 20kgs. Others, like me, who start off overweight don't necessarily put on a lot.
Then, like Sarah mentioned, there's fluid retention, the size of your baby etc. etc.
I really am in the belief that watching your weight while you are pregnant is not good for you at all. It should me more about how you feel. Listen to your body. If it is saying you are hungry, then you are! If it is saying you are full, then stop.
With my next pregnancy, I am going to ask my Dr. not to weigh me at all. The midwives at my hospital have stopped weighing women at every visit because of the negative impact it has on the mind-set of the ladies.
Worry about it afterwards when you can do something about it, I say!