Numerical errors usually occur for one of these five reasons:

• A journalist mishears a correct number given to them by a source and fails to double-check it.
• A source unwittingly provides a mistaken piece of information and the journalist fails to verify it.
• A source deliberately fudges the numbers and the journalist fails to verify them.
• A journalist or editor miscalculates a figure.
• A journalist re-reports a mistake made by another media outlet.

Columbia Journalism Review