You frame has "compact geometry" which is more common these days than traditional geometry frames with horizontal top tubes. Look around you, most bikes are like this. There's nothing wrong with it and there's nothing you can really do to correct it without messing up the fit and handling characteristics of the bike. It will ride and fit like a bike with a horizontal top tube...the only real difference is that you have to use a longer seat post. If the look of it bothers you, I suggest you just get used to it.