Why prod is used at staging server?

As we deploy to the staging server as:

mix edeliver update staging --branch=master --clean-deploy

we notice that the server is started using the settings in the prod.exs file, instead of the test.exs file.

Any idea?

Usually you want your staging server to behave identical to the production server, because this helps ironing out bugs before they happen in production.

test.exs is used as config when running mix test.