This is most likely the edition you have. Scribner's put out the last edition of the six-volume set in 1922, revised and updated by colleagues (I see James Alton James [1864-1962], then a professor at Northwestern, sometimes attached) under the credited copyright of Ella A. Andrews. She was the widow of Elisha B. Andrews, as shown on the copyright catalog's page 1398 here. As to the reason for this edition, it was likely demand (the set was Andrews's magnum opus, and to bring it up to the war and its aftermath was valuable for Scribner's) and possibly the likelihood of royalties for Ella Andrews and/or other collaborators despite the fact that, as a former president of Brown, Elisha and Ella were both probably born into wealth and stayed there. I don't know much about her qualifications and connections, but the American National Biography reference essay on Andrews may talk about her as well. Unfortunately I don't own a set of those, but they are widely available references at major libraries (sometimes online). If someone can find a volume I of the 1922 edition online, they may say more in the foreword, preface, or introduction; I was unable to locate one. edit: I finally found one. It explains in the Publisher's Note on page 6 that James Alton James updated the volumes through Taft, but that Paul L. Howaaworth - about whom I've found little other information in my cursory look - did the final revision for the 1922 edition.

edit: Added James's dates; Haworth; etc