Why isn't the sum of the masses of the subatomic particles of an atom equal to the mass of an atom?

Hydrogen is..

1.008 grams/mole

That is 1.008 grams is 6.022 X 10^23 atoms of hydrogen.

So, 1 atom of hydrogen..

1 atom H/6.022 X 10^23 atoms

= 1.66 X 10^ -24 grams
===================the mass of one atom hydrogen. Check that against the one electron mass and one proton mass of hydrogen and see if there is a variance. If so, check back.