When not the latest version of core is installed, the translation update system will download and import translation of the latest drupal core version. This behaviour only applies to core and not to contributed modules. This may cause confusion when the translation version is compared to the version of the installed core release.
As a side effect the test scripts which test the translation update code, must do additional effort to ignore the core release. Since the test scripts can be run with various core versions (currently 8.x-dev, but in the future 8.-RC1, 8.0, etc.) this unstable factor must be eliminated. The above behaviour makes this elimitation more complicated, as was discussed in #26.
This problem has been reported before in the Drupal 7 branch of Localization Update module. See
Remove a piece of code which always applies the latest core release version to the core project.
User interface changes