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I think the decreased fuel usage at higher speed might be an anomaly in data collection or something. If you look at a BSFC chart that has lines going all the way to the 600g/kWh mark, and plug and chug some numbers, you'll notice that the additional fuel used when adding load at idle can be even less than additional fuel used adding the same amount of load in the higher efficiency islands. This makes sense, because when the car is under very low load, an increase in power demand only takes a little bit more fuel because you're derestricting the engine, allowing the combustion to take place at higher temperature, etc.
In other words, the efficiency increase from adding that load is enough to make the marginal fuel usage small. For the same reason that reducing your drag by 10% doesn't increase your mpg by 10%, increasing load by 10% does not decrease your mpg by 10%.
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