That is an AUR package. That means you build it on your machine and as a result it uses the Python version you have on your machine at the time.
powerline is a repo package. This means the maintainer builds it for you and it will install the files where they are in the package.
However, python-powerline looks to be correct to me in the repos. This probably means you have the wrong version installed. The current version is 2.8.3-2