"Accounting is not fully exact"- PLease explain the statement

Even though all the business transactions entered in the books are based on proofs but still there are certain transactions that are recorded on the individual judgment of the accountant. For example, method for valuation of stock may differ from person to person. Similarly, method of providing depreciation can be varied. This will lead to the different financial results by the different people and thereby the profit cannot be considered as accurate profit. Hence, it is stated that accounting is not fully exact.