For a detailed listing of the Library of Congress's classification numbers and their descriptors, see the Library of Congress Classification PDF Files, last updated in February 2018.
In the Library of Congress Classification system, the range PN6700-6790 is assigned to "Comic books, strips, etc.". Inside that range, the subrange 6725-6728 is for "United States". I assume they put The sandman. Vol. 1, Preludes & nocturnes / Neil Gaiman, writer (...). New York : DC Comics, c2010. because it's a comic published in the United States, Sam Kieth, Malcolm Jones III and Todd Klein are/were an American artists, and Mike Dringenberg appears to be working in the USA.
American Gods, by contrast, is a novel published by an English author and classified at PR6057.A319 A84 2004. Wikipedia says the novel was first published in 2001; the Library of Congress (LoC) owns a reprint from 2004. Neil Gaiman was born in 1960. The range PR6000-6049 is for English literature "1900-1960", the range PR6050-6076 (where they put the novel) is for English literature "1961-2000" and the range PR6100-6126 is for "2001-". The novel is in the range PR6050-6076 because the "[p]eriods reflect the time period during which the author was productive – not when he/she was born" (quoted from Fundamentals of LC Classification: Instructor Manual).
As to why comics are under PN and not under PR, that is not so clear. However, the Fundamentals of LC Classification: Instructor Manual (downloadable from this Catalogers Learning Workshop page), says that the LCC is "too large for an individual to fully master" and that "parts of its organization still reflect 19th/early 20th century worldview". In addition, Wikipedia points out that, "LCC has been criticized for lacking a sound theoretical basis; many of the classification decisions were driven by the practical needs of that library rather than epistemological considerations."
Others have also pointed out that classification is not always based on logic. See, e.g. the article 'Whaddaya Got?' Finding Graphic Novels in an Academic Library by librarian Karen Green: "(...) The Library of Congress has three separate approaches for classifying graphic novels and comics. (...)"