Speed increasing with 2-bath developers

I exposed a roll of Ilford HP5+ exposed at ISO 200, 400, 800 then developed the entire roll in a 2-bath Diafine developer for 3 min in each bath. The 400 and the 800 appear slightly overexposed with good defination, but the 200 is obviously underexposed with increased contrst. Is this typical of 2-bath developers? How is the apparent speed increse achieved? Any try pushing to 1600 and beyond?

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