The issue is with books that contain several parts of the series. See https://app.thestorygraph.com/series/1228267 and https://app.thestorygraph.com/books/cb07c7df-594f-4e71-8b70-c5244e8e09c4 for example. In that series the first eigth parts are novellas that are published as four books (with two novellas in each), but the last book in the series is a novel. So we have four books that have #1-2, 3-4, 5-6, and 7-8 in their series counter and then we have the ninth book as #9. But when you search the series by name the only book that shows up is the #9 - the novel.