Tax returns

I was employed for almost 15 years as a tax payer. For 5 year my tax was not less then R1500 and 4 year R1990 above during overtime. Now my problem is when I was dismissed and I did my text return I only received R12000 for my taxes returns and you some who received R17000 for working 4 year and earning less that me when it come to wage or salary. How is that possible?

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