The ‘which’ and ‘where’ parts of the question were answered by Maupassant’s biographer Francis Steegmuller:
A smart American publisher and bookseller named M. Walter Dunne, whose specialty was large and showily produced “sets,” felt that the time had come to confront the American public with all of Maupassant; and he issued [starting in 1903], in seventeen volumes, the first large-scale collection of the stories, novels, travel-writings and verse in English. […]
But the most remarkable feature of the Dunne collection is its contents: for in it are included no less than sixty-five stories which are not the work of Maupassant.
None of these sixty-five stories has ever been included in any French edition of Maupassant’s works; I have been able to identify four as tales from the collected works of one of Maupassant’s fellow-journalists, René Maizeroy; the authorship of the remainder is unknown; but almost without exception internal evidence shows them to be non-Maupassantian. Falsity is particualrly obvious in those dealing with Central and Eastern European life, especially Vienna and Budapest society, territory never explored by Maupassant, and here described in a very non-Maupassant style. Almost all of the sixty-five give the impression of having been trash in their original language or languages, whatever it or they may have been.
The Dunne collection was followed, during subsequent years, by many smaller, less elaborate Maupassant collections for smaller purses, and although Dunne's collection was copyright it seems to have been without real protection, for in the later collections various of the fake stories first published by Dunne continued to appear and reappear.
Francis Steegmuller (1949). Maupassant: A Lion in the Path, pp. 355–357. New York: Grosset & Dunlap.
The following list gives, for each of the fake Maupassant stories, its English title (or titles; some stories were reprinted under different titles), its true author, its original title, and the collection in which it appeared. The list was compiled by Kazuhiko Adachi (足立和彦) and contains the 65 fake stories found by Steegmuller, plus one more (‘Mad’) found by Onishi Tadao (大西忠雄). The stories are by René Maizeroy (28), Jean Richepin (20), and Leopold von Sacher-Masoch (18).
To answer the ‘why’ of the question is probably impossible now, but it is easy to imagine plausible scenarios. Perhaps the publisher had employed a translator in Paris to go to the archives of Gil Blas, La Lanterne and so on, dig out old stories by Maupassant, and submit English translations, at so many cents a word. An unscrupulous translator might have found it tempting to eke out a little extra cash by passing off some stories by other authors. Or, more likely, the publisher deliberately included the more risqué and sensational stories of Maizeroy, Richepin and Sacher-Masoch, in order to increase the appeal of the collection.