Also, OpenAPI is based on JSON Schema, the currently published version, 3.0.3 is based on an older version of JSON Schema that has a couple of small differences with the latest versions of JSON Schema. The latest RC version of OpenAPI, 3.1.0, is built on the latest JSON Schema version.
FastAPI is built on Pydantic, that generates JSON Schema with the latest versions. So, the OpenAPI generated by FastAPI has some differences in a couple of corner cases where it is compatible with the latest RC version of OpenAPI. The advantage is that the internal JSON Schema declarations use the latest version, that are compatible with recent tools that use them, e.g. automatic frontend components.