Sky Digiboxes

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I've had my bog-standard Sky digibox replaced earlier this week by a Sky engineer. I telephoned Sky on Monday and the box was replaced on Wednesday. The engineer told me the replacement box had been delivered to him on Tuesday and he knew I was the customer as I was his only job not involving a Sky HD box!.
Last night, the replacement box broke down. I telephoned Sky this morning, complained bitterly that I had paid £65.00 for a rubbish service. I was eventually told they would pull out all the stops to get an engineer to replace the faulty box tomorrow (Saturday) afternoon.
If Sky need a day to deliver a box to an engineer, how can they suddenly find an engineer with a replacement box in stock on the day after the complaint? How does this system work?

Thanks