Initially PD4ML renders a document as for a screen representation (converted all physical units to screen pixels with 72dpi). After the layout is rendered it is stretched or shrank to match the target paper page width. See

if you need to keep physical unit dimensions to exact on the paper, you either need to play with htmlWidth parameter or to trigger pd4ml.protectPhysicalUnitDimensions().

(A drawback of the API call is a possible misbalance of resulting sizes of objects whose dimensions given in physical units and ones defined in screen pixels)