Weighed feed?

Question I have always struggled with low supply and my now 8 month old has basically been supplemented since birth. When I pump at work I am lucky to get 4 oz combine for first pump, 3 for the second (most was 5 oz right at 4 months when he started sleeping 8 hrs instead of 6. Well this morning I changed his diaper weighed myself, and then held him and stepped on the scale again. He was 15.8 lb. I did this same procedure again after nursing and got he weighed 16.2 lb. our combined weight was exactly the same which makes sense since the milk left me and went to him but does this mean he actually nursed 6 oz? I honestly figured he was getting about 4 total. When I pumped before work I only got about 3.5 oz. thanks