Why in case of synthetic polymers molecular mass is always expressed as average molecular mass ?

Explanation

Let `N_1,N_2,N_3`……………… are the number of molecules having molecular masses `M_1, M_2,M_3, `…………….
`bar(M_n) = (N_1 M_1 + N_2M_2 + N_3M_3+…….)/(N_1 + N_2+ N_3+……) " " `bar(M_n) =(sumN_1M_i)/(sumN_i)`
Here, `N_i` is the number of molecules of `i^(th)` type having molecular mass `M_i`.