Mass of an atom v. Mass of constituent subatomic particles

[Arianna Zhou 1D]

Hi, post-exam I realized the way to calculate the mass of an oxygen atom is by dividing its molar mass by Avogadro's number: M / N_A = 16.00 g/mol * (1 mol / 6.022 * 10^23 atoms) = 2.657 * 10^-23 g per atom of O.

I was wondering why this number is smaller than the mass of 8 protons and 8 neutrons, since: 8 * (1.673 * 10^-27 kg) + 8 * (1.675 * 10^-27 kg) = 2.678 * 10^-26 kg = 2.678 * 10^-23 g.

[Vyas_Koduvayur_2B]

I think it's just due to approximate values. 8 protons + 8 neutrons is about 16 amu, which is 2.6569 * 10^-23 grams