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This is the same engine that was in my Dad's Focus Diesel back in the day.
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It could likely be due to the characteristics of the torque curve. If I recall, the boost starts building between 1,500 rpm and 2,000 rpm... at over 2k, the turbo is spooled and the engine is in its sweet spot in terms of power production and BSFC. (Saw something similar with the lower-powered VW 2.0 TDi... it's actually slightly more economical at a higher cruising speed in 6th because of the turbo.
1,700 rpm might just be where the engine enters enrichment mode... which is why the readings are so high.
One thing you could try is to map out this section of your powerband more thoroughly, between 1,500 and 2,000, to see exactly where this happens.
Then just avoid driving in that rpm range when you're out on the road.
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