First the why 7 cell part. In general the wing shapes that are closer to square as opposed to a long rectangle are less subject to some malfunctions. The second about f-111 fabric. Reserves as opposed to mains must meet rigorous certification requirements. Those requirements as to load and speed at deployment make it harder for z-po fabrics that trap more air to both open quickly enough and maintain structural integrity. “How are Maxwell House coffee and skydivers alike? Well; their both good to the last drop.” - George