The relative atomic mass represents the average mass of all isotopes of an element relative to the mass of carbon 12 isotope. The evaluation of relative atomic mass depends of different masses of isotopes of the element and the percentage of abundance of each isotope in element.
As you can notice, the relative isotopic mass cannot be compared to the relative atomic mass, whose formula of evaluation is the following:
`R.A.M = (a*m_a + b*m_b + c*m_c)/100`
a,b,c are the abundances of each isotope A,B,C, of an element
`m_a,m_b,m_c` are the mass numbers of isotopes A,B,C of an element
Hence, there are three isotopes of Argon: 36,38,40. The relative abundance of each isotope is: 0.34, 0.06, 99.6. The mass numbers are: 36,38,40.
Evaluation of relative atomic mass is:
`R.A.M = (0.34*36+ 0.06*38+ 99.6*40)/100`
`R.A.M = (12.24+2.28+ 3984)/100 `
`R.A.M = 39.9852`