# Why is it necessary to use the average atomic mass of all isotopes, rather than the mass of the most commonly occurring isotope, when referring to the atomic mass of an element?

Why is it necessary to debit my account by £7-49 instead of £7-50, even tho' the $\text{1 p}$ difference is unimportant to me?
PS If my account were debited £7-50 instead of the £7-49 purchase price, I would raise merry hell, and demand satisfaction from my bank manager.