The 6.5 minutes is the time you should leave the film in the developer when using the stock solution. The 9 minutes is the time you should leave your film in the diluted solution. It takes longer to develope the film in the diluted solution because, well, it's diluted. I don't know how over developing film affects the film because I have never done it. One really nice thing about traditional b&w films is that they have great latitude. Like AV said, your film is prop. over developed, but I'm sure you can still get some decent prints out of them.