Basic / Higher Rate of Tax (Saving Income)

Came across the following example: Questions: Taxable Non-Saving Income: $33,950 Taxable Saving Income: $5,550 Total Taxable Income: $39,500 Solution: Non-Saving Income Tax = $33,950 x 20% Saving Income Tax = $500 x 0% (Saving Income Nil Rate Band) = $3,050 ($37,500 - $33950 - $500) x 20% = $2,000 ($$5,500 - $500 - $3,050) x 40% My question is why did the author use the higher rate taxpayer of saving income nil rate band ($500), when he use the basic taxpayer rate of 20% in the non-saving income. How do I justified whether an individual is a basic OR higher rate taxpayer when applying the saving income nil rate band?