This is a problem that affects gxine and, I believe, Hulu desktop, among others. These programs have windows with fixed aspect ratio, and when I try to resize the fixed-aspect window, using the grips, the sizes are miscalculated.
The attached patch seems to help.
(Personally, I would like it better if aspect ratio were not updated during the resizing operation, but only after I let go of the grip. That way the grip feels much more like something on which I am grasping, and the quick resizing that occurs after I let go also feels natural, like putting an elastic object into something. So I'm thinking of commenting out the pertinent code, in my own copy of fluxbox.)