The title “General Counsel” has a longer lineage than the modern “Chief Legal Officer.” It would be interesting to find the earliest internet reference to CLO.
Where a company has groups of lawyers reporting to a business unit, such as Nokia and its eight legal departments each headed by a “Vice President, Legal,” then the company often distinguishes the top lawyer with the sobriquet “Chief Legal Officer.” (I have never run into a Chief General Counsel.)
CLO also elevates the legal leader to the so-called C-Suite, the titular peer of the Chief Financial Officer (and why not the “General Bookkeeper”?), Chief Marketing Officer, and Chief Technology Officer (so where is the Chief Human Officer?) and others in the executive teepee.
Finally, GC implies the common array of legal functions – litigation, corporate secretary, and law-school course counseling. Chief Legal Officer connotes to me a broader portfolio, such as compliance, risk management, ethics, lobbying and the like.