PostgreSQL claims to support 170 of 179 SQL:2016 features, but it also lists 309 features of SQL:2016 that it doesn't support. Why the discrepancy?
Features in the SQL standard are split between mandatory features and optional features.
As stated here

Out of 177 mandatory features required for full Core conformance, PostgreSQL conforms to at least 170. In addition, there is a long list of supported optional features

(emphasis mine)
So the discrepancy you are seeing, stems from the difference between mandatory and optional features.