Because in old IE, <h3> was defined with an absolute font size in the built-in style sheet, in newer browsers it is defined as a relative size. Since we have nested <h3>'s they get increasingly bigger in newer browsers, but in old IE they would all have the same size.
It's because some browsers close the currently open heading tag when they encounter a new one (since these tags can't be nested) and others do not. Since the text size is applied as an H3 style and is relative to the parent, the nesting of these tags (or not) determines whether the effect of the style is cumulative.