Jim - this is to do with the fact that a button when clicked is calling up an overlay image, which consists of just four colours... or is in four shades of grey. There is precious little control over the quality of the font outline at that point. This is why a lot of menus don't use text in their highlight images, but do underlines or similar to indicate what is being chosen. The text exists on the menu background in these cases, and so can be as detailed as you like.
Other tricks are to set the highlight up so that it appears to 'fill' the text on the background, but not to the edges... the background is slightly larger and so the fine quality edge remains even when the text is selected.
There are lots of tricks like this that work around the shortcomings of the DVD specification... just a matter of finding the one that works best for you in your situation. Personally though, I'd avoid using text in a button highlight for anything other than a test disc or rough demo or concept.
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