Why is there a default boost of 2.2?

[slumx (Peter B. Vahlstrup)]

In Elasticsearch 7.1 when no boost is added (index or query) the explanation still says there is a boost
of 2.2.

Steps to reproduce:
follow the steps described here
https://www.elastic.co/guide/en/elasticsearch/reference/current/getting-started-create-index.html
and here:
https://www.elastic.co/guide/en/elasticsearch/reference/current/getting-started-query-document.html
and do a query with explain

GET /customer/_search?pretty=true&q=doe&explain=true

Is this 2.2 some kind of base boost or where does i come from?

The 2.2 also appear in the documentation for explain:
https://www.elastic.co/guide/en/elasticsearch/reference/current/search-explain.html

[slumx (Peter B. Vahlstrup)]

Figured it out, i think?. It seems that lucene removed (k1 + 1) from the bm25 similarity, so this os added as a boost instead in ES.
see https://issues.apache.org/jira/browse/LUCENE-8563