I think from what Becca said above all the paragraphs are enclosed in blockquote tags. Does the Kindle "hard code" a margin for a blockquote if it is not specified on the style?
You could just open the ePub in Sigil and "blow away" all those <blockquote> tags - so the <p> tags are using only whatever class style is associated with them (which following the advice from JSWolf should now be zero for left margin).