Why is there a default boost of 2.2?

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
and here:
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:

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

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