Tax allowable depreciation, double taxation treaty

Hi,

If there is a double taxation treaty and extra tax is payable in the UK (e.g. 10%) on the taxable profits, why do we deduct from the taxable profits the tax allowable depreciation, reducing balance, in full?

EG the investment was 580m reducing balance 4 years. in Year 1 Kaplan Exam kit Exercise 2 is reducing -145m (580m * 0.25), instead of -43.5m (580m * 0.25 [reducing balance rate] *0.30 [UK tax rate as per the question]).

Is this right because of a singularity of double taxation treaties?

Thanks in advance