Why pay tax every month but when you submit your returns they sms you that you still owe the tax man

Why is that even though you pay tax every month and you submit your tax returns they still tell you that you owe tax man a certain amount. How do they got to that conclusion?

It all depends on whether enough tax was being deducted each month and paid to SARS and if you earned any additional income. Please make use of our SARS income tax calculator to see what the tax should have been.