Audit opinion – past paper june/07

Hello, could you please explain the audit opinion to me, Im not getting how is this inability to obtain sufficient appropriate audit evidence?

Q-MSV
(b)
Issue 1-
” Audit tests on sales indicates a deficiency in the internal control system, with a potential understatement of income in the region of $500,000. The deficiency occurred because sales invoices are not sequentially numbered, allowing one of the directors to remove cash sales prior to recording in the sales days book. This was identified during analytical procedures of sales, when the audit senior noted that on the days when this director was working, sales were always lower than the days when the director was not working.”