The terms centralized and decentralized are used with two unrelated meanings. To many people who describe law department structure, they refer to the geographic location of lawyers (together physically at headquarters or spread around).
For others, it means the reporting relationships of the lawyers. In the second sense (which I use), a centralized department has nearly every practicing lawyer, no matter where they are physically located, reporting to the general counsel, while a decentralized department has at least some lawyers reporting to the heads of business units.
Confusion follows from people not defining their terms, and from the obvious fact that all the lawyers in some far-flung, multi-location law departments ultimately report in to one lawyer and the equally obvious fact that in a few legal departments there may be many lawyers under one roof who report to different business executives.