WHY AM I OWING SARS THOUSANDS EVERYTIME I SUBMIT BUT HAVE NO BUSINESS?

I AM A GOVERNMENT EMPLOYEE SO TAX IS ALWAYS DEDUCTED FROM MY SALARY I ALSO HAVE EXTRA INCOME FROM MY LATE HUSBANDS PENSION FUNDS ALSO TAX IS DEDUCTED THERE BUT ALWAYS I'M OWING THEM THOUSANDS OF RANDS EVERY TIME. WHY? I DON'T UNDERSTAND BECAUSE I'M ALWAYS SUBMITTING

The two incomes are not added together when the companies take off the tax on each income. They treat it as if there is no other income and therefore only tax it on one income. However when you submit your return you have all your income added together and this causes you to owe even more tax unfortunately.