Its about payment. Do you think importers from other countries just pay the money straight up. They take time. Also its not constant inflow/outflow.
Say, KSA has to pay Pakistan 800million, and we have to pay 400million. Instead of two transaction, we just ask KSA to transfer, 400million to us. Also look at the timing of the deposit, last week. It the same time, whole months export/imports are calculated, and then a single transaction is done.
It doesn't even have to be for 1 month, it could be for 2 months. And the government does operate on accruals bases. The whole world does. If its didn't, then reserves will be falling/rising everyday, not weekly or monthly bases.
Don't look at just one area of industry to generate 400million. It could be multiple, food, textile, meat, etc. Maybe all of this can add up to 400million in one or two month.