[Caption] code adding inline style of 10px

Just bumped into this myself. It’s being discussed here:
http://core.trac.wordpress.org/ticket/14380

Looks like a fix is on the way for the next version, but there seems to be some disagreement over how it should be done. There’s a reason why WordPress shoehorns an inline style in there (see trac ticket), but it’s an awful solution, there must be a better way.

There’s no clean and simple way to override this in your theme’s stylesheet (you could target .wp-caption.alignleft in your stylesheet and give that a width of 300px !important, but ONLY IF you always insert left-aligned images at the medium size). It’s not even a fix, more like an awkward temporary workaround.

I’d suggest living with it for now, and see if there’s a fix in WordPress 3.2. Or you can try stuff like this:
http://wpengineer.com/917/filter-caption-shortcode-in-wordpress/
I haven’t tried it yet, so I don’t know if it catches all instances etc.