All the explanations are also in the descriptions but to sum it up:
Due to being caught up to the manga, first season of Kuroshitsuji had an original non-canon ending, which then was followed into the second season: Kuroshitsuji II.
Few years after those came out, we got "Book of Circus" which completely ignored all of the anime-only content from the previous two seasons, and instead simply adapted the manga arc that happens right after episodes 13-15 in Kuroshitsuji.
Because of all this mess you now get a choice of skipping the non-canon episodes and only watching those that were adapted from the manga. Whether you /should/ do that is all up to you. People generally prefer the manga adaptations, but Kuroshitsuji II also has a fair share of fans and introduces new characters that some people like.
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