For example let’s take the oatmeal that might have 17of protein, 1g of fat, 40g of carbs… the caloric total would be:
17 x 4 + 1 x 9 + 40 x 4 =
68 + 9 + 160 =
237 calories
If we count only half the protein total we now count 8.5g of protein. Since each gram of protein protein provides roughly 4 calories 8.5g of protein would be 34 calories. So if you were to only count half the calories from protein (since you are only counting half the protein toward your total content) the caloric total would be 203 calories instead of 237.
But an incomplete protein still provides the same “energy/calories” as a complete protein so we can’t use half of the amount to get your caloric intake.
So even though you are only adding 8.5g to our protein total we still use the full 17g to count the caloric content of the oatmeal.