Almost all of Mavis Gallant's stories appeared in the New Yorker, which is the explanation for—or perhaps shorthand for—both their strengths and their weaknesses. The Collected Stories, put out by Knopf/Everyman in 2016, perhaps to acknowledge Gallant's death in 2014, is identical in content to the Selected Stories put out in the UK by Bloomsbury in 1997, and the British edition has a preface by the author that is worth considering. (The introduction to the American edition is by Francine Prose.)
In this preface, Gallant calls her association with the New Yorker a "wave of the best luck." And so it clearly was, as they seem to share many qualities in common. The New Yorker is smart, and so are these relentlessly well written stories.