chapter 5 lecture PREPARING the capital allowance computation, NOON POOL ASSETS

hallo,

In example 2 in this lecture we deduct a disposal of 9400 from 13600 and up to this moment is fine for me. What i do not understand is why after that we did not calculate WDA for this what left.
13600 cost – 9400 disposal = 4200
4200 x 70% = 2940
WDA 2940 x 18% = 529 and this add to allowances column

if you can re-explain(missed and could not find it) i will be very thankful

Mateusz