Thanks very much for your answer! I must admit I had to read it few times but it makes sense now.
However I tried to play a bit around and I came to this conclusion: the extra lines are generated by Kindleunpack, not Kindlegen. If I send the Mobi file to KindleUnpack I get an HTML file which (when opened using Chrome, or even Calibre) shows already the extra lines between paragraphs - extra lines that were NOT there in the original Mobi.
Am I adding confusion?
