Direct tax section 32

For air pollution control equipment the depreciation rate is 40% and so additional depreciation of 20% is also claimed. Now in new study material for May 2018 Illustration 4 Profits and gains from business or profession addiitonal depreciation is charged twice once in the 15 % block and next in 40% block. Why is it so? Csan please somebody clarify?

https://resource.cdn.icai.org/46588bosfinal-p7-cp6.pdf?newbg=true

Thanks

Indira K V