It must be that legal work done in-house has steadily moved up the value curve much like the work done by outside counsel has elevated (See my post of March 11, 2011: huge productivity increases, plus quality.).
By “value curve” I mean the importance of the work done for the company per hour of lawyer time has increased, however we measure that. In general tasks have trended up on complexity (multiple inter-related issues), urgency (documents or advice needed quickly), relations (more people involved and requiring consultation), consequences (resulting profits or risks avoided are greater) and information (statutes, regulations, law reviews, cases, online information, press and commentary, internal documents). Value delivered is a function of those five components.
Both in-house and outside, commodity legal services – those of relatively less value as defined — have drifted away to other providers or become so standardized that they take less attention and have been delegated or have been handled by better informed clients. This upward migration of work may account partly for why the number of lawyers per billion of revenue has remained relatively stable for years. The lawyers are solving higher-level problems, doing more important work, as well as performing more efficiently.