One reason is if it is being resized for display in the forum then that will effect quality. The display size we have set here on this forum is 50px – so if your gravatar as stored is bigger/smaller then you must expect some degradation as the browser manipulates it. We do have ours set for a 99% quality resize but this will still cause images to degrade a little.
Another possibility is if you have changed the original at any point for a sharper version. We cache gravatars here (using the SP plugin) so if this were the case we might still have an old version cached. If this were the case then you can reset this on your forum profile which will destroy the cached version and force the latest one to be downloaded next time it is displayed.
But the first reason is more probable.