The DPT-RP1 fits pages to its screen using each page's height. Your page A has roughly the dimensions of the DPT-RP1's screen, so it renders A's margins correctly. Your page B is taller, so the DPT-RP1 adds width to render it with the DPT-RP1's dimensions. This might not be a problem except that the DPT-RP1 does not remember page crop settings (and many features are not available in cropped-page mode).
It's a major, software flaw with, apparently, no workaround within Sony's software. You might post this to Sony's moderated, community webpage to see when this flaw will be fixed.
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