Compare the images. Notice the fonts are different?
(http://i47.tinypic.com/2627bki.png)
(http://i49.tinypic.com/2vl7400.png)
Been that way for as long as I can remember. The Mantis integration is... flaky.
When this site had the former sitetheme on, before Curve was used, I had similar problems adjusting Mantis CSS to suit the sitetheme then - so I am not surprised its a difference. 2 different scripts and different native css will, unless totally stripped for similar style declarations, clash on stuff.