The semantics are important. A release is composed of multiple distributions (sets of files). Each distribution does have a one-way hash value. This is what you lock to. Adding files does change the value of that hash. Files cannot be removed or changed.
You lock to the distribution instead of the release so your build doesn't have to download distributions for platforms you're not using.
You lock to the distribution instead of the release so your build doesn't have to download distributions for platforms you're not using.