You spent a considerable about of time working on designing an aesthetically pleasing email. However, it looks different on Outlook 2010 vs 2013.
HTML and CSS work more like suggestions. These two web coding languages are not programming languages, so there is room for interpretation by the engine that renders it, such as a browser or email client. While there are standards set forth by W3C, there is room for flexibility.
At a high level, it is expected for emails to look different, even within the same platform such as Outlook 2013 vs Outlook 2010. With that said, you can code against some of these anomalies by adding extra code in CSS to account for the differences. You should consult with an HTML/CSS designer to make these changes.