They have significant, complicated exposures off the books that need to be considered more explicitly in determining minimum regulatory capital. Among these are operational risks, which in some recent instances have been the source of substantial losses far exceeding credit- or market-related losses.
Basel II, by tying regulatory capital to bank inputs, offers greater transparency about what stands behind the inputs provided by banks and exactly how they are calculated. As I noted earlier, successful institutions understand that calculating internal capital measures is a serious and challenging undertaking, with many moving parts.