I am not sure how the Castle Garden site has organized its lists, but they apparently have many whch lie outside the years of Catle Garden's opeartion and some that were not even New York arrivals.
The reason you cannnot find Julia Shea's passenger manifest is because the voyage of the Samaria in September 1872 was Liverpool to Boston.
Here is the manifest at FamilySearch.
https://familysearch.org/pal:/MM9.3.1/TH-1-17919-19886-99?cc...