Found this: http://wordpress.stackexchange.com/questions/178514/the-content-returning-unexpected-div-wrappers
It seems that this can be cause by copy and pasting. I have see this before, for example if you go to this site http://lipsum.com/ and generate some paragraphs of text and then copy an paste all of the text into the visual editor in WP you will end up copying and pasting the surrounding div as well
<div id="lipsum">
.....
</div>
