I see. It's something that's going to be more apparent with image posts because of the shorter text line, but that's something I have noticed also - the preview display line-length is different than the actual display line-length.
I'm not sure why that is, but if it's something that's essential to the formatting of the post, perhaps using hard returns instead of relying on word-wrap might be a work-around? In my case, it's never come to that level of precision.
EDIT: To clarify for others - when I did a preview on this post before editing, the line broke at 'line-length', and 'of precision'.