dec 2013 Q1 working capital calculation

Just working the question and I cant understand why the working capital is calculated in this manner, I have tried working through the answer but still don’t understand;
it says on the answer
” the working capital cash flows are timed differently to the sales income on which they depend, and so their inflation effects are timed differently to the related inflation effects in the discount rate”

please can you explain? why are they timed differently and how do you calculate it? thank you 🙂