I think the quick answer is that's the way your built. Also, low bf or not, that's a place where the body tends to hold onto a little fat (and also water). Which means for some you have to get it down to a very low single digit number (which I think is a waste of time). If you're already 9-10 then I'd say screw it, work on getting as wide and thick as you can and you won't be sweating an 8 pack anymore
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Even then there just may not be as deep a ridge in that region. If you look around at different people who are very ripped you will notice plenty that may have a very cut 4 pack, or 6 pack...but you'll rarely actually see an 8 pack. Even in those cut guys, some of their lower abs just look like one solid plate. It's individual variation and you'll get what you'll get.
Another thing to consider is that sometimes after you consistently train your abs for a while and do heavy weight lifting, depending on the individual, the transverse abdominus can get relatively weak (it's not visible). This is what sort of holds your abs in making them appear flatter and tighter. Remember there is not bone under you lower ab region for the muscle to hug to. The end result is a little more bulge.
Personally I do weighted ab exercises (sort of a standing situp with weight) most of the time. I spice it up with rotational stuff and try to keep my obliques strong. But while I do the excercise I always hold my abs in a vacumn, i.e., suck the lower abs in as if trying to draw them thruough the spine. That'll strengthen the transverse and make your lifting all the more safe. I was getting a bulge but now that has taken care of it and the only bulge I have is the little bit of fat I like to keep on. It's hard at firts but once you get in the habit it's easy to incorporate.
Hope some of that helps.