Pacific Islander vs Asian isn't "fine-grained". They are two distinct "buckets" in the US government's standard racial classification system, to which I linked earlier [1]. It is just that the "Native Hawaiian or Other Pacific Islander" bucket sometimes gets its name abbreviated to "Pacific Islanders". "Pacific Islander" is a collective term referring to the indigenous peoples of the islands of the Pacific Ocean, which peoples are divided into three main ethnolinguistic super-groups – Polynesians, Micronesians and Melanesians – and Native Hawaiians are a Polynesian people. New Zealand's Maori are another Polynesian people.

Prior to 1997, the US government had a single bucket, "Asian or Pacific Islander". Since 1997, they've been split into two separate buckets (although it took some time for the bureaucracy to fully adopt the new grouping – the 2000 census still used the old classification, but the 2010 census adopted the new one.)

[1] https://www.census.gov/topics/population/race/about.html