Please clarify the license for boostbook/xsl/caramel

The directory contains this license file, which seems to be a form of the [MTLL](https://spdx.org/licenses/MTLL.html) license:
https://github.com/boostorg/boostbook/blob/develop/xsl/caramel/LICENSE

However, the other three files in that directory all say:

Distributed under the Boost Software License, Version 1.0.
(See accompanying file LICENSE_1_0.txt or copy at
http://www.boost.org/LICENSE_1_0.txt)

So which is it? If those files are distributed under BSL-1.0 then why is the other license file there?

This complicates things for downstream packagers like Linux distros, who need to audit and record all relevant licenses used by a package.