Would need to add bomb body breakup, fuze chain reliabilty, and low order detonations as a chance...When you start dropping bombs that skip...weird stuff happens.
http://www.researcheratlarge.com/Aircraft/1943TacticalBulletins/3-43_MastheadBombing.html
1. "One heavy bomb group in the Fifth Air Force has employed high level daylight bombing at 6500 to 7500 feet on practically all targets (land and sea) and has achieved remarkable results. Their pilots and bombardiers conduct bombing training at every opportunity and have developed an anti-aircraft fire technique, that, to date, has been successful in that no aircraft have been lost due to AA fire. However, AA fire has scored hits on their aircraft. Their method briefly is to approach the target about 2000 feet above the proposed bombing altitude. When about 10 miles from the target RPM's are boosted to about 2250, throttles closed to l5"Hg to 18"Hg and a 180 mph, 1000 ft. per min. descent started. This descent is maintained and so jockeyed that when the bombing altitude is reached (usually 600 ft.) they are ready to start their bombing run. Normally about a 70 second run is employed with a sharp diving turn when bombs are away. The undersigned was on one such mission where moderate AA fire was encountered. All of the first burst broke over and behind. By the time the AA started coming extremely close we were about out of range."