Simple you have to diet hard to get below 10% bodyfat. When your goal is to be strong and add as much muscle as possible you are going to add some fat along the way. Also a little extra bodyfat also helps your joints by adding more water into your body to help keep them lubricated. Also in order to store more glycogen for muscular effort you can't limit your caloric intake as much. Furthermore many powerlifters look fat in regular clothes because of the shape of their muscles. They want a bigger waist with more muscle, as opposed to the thin tapered bodybuilding waistline, so they have more stability. If you see Ed Coan, Chuck Vogelpohl, Brent Mikesall, or Scott Mendelson with their shirts off you wouldn't call them fat. Their abs bulge out but are very well defined due to their style of training.
Cheers,
Scotsman