I'd like to know that, too. It seems to happen with the xml text field only. Also, my <br />s in templates are changed to <br>s, which I'd also like to change, as I want to use XHTML...
Then you'll see which template is being used for displaying your text. Alter this file, or create an override for it, and your design will be fine. You'll probably find the file in the "design/standard/templates/content/datatype/view"
