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After the release of 7.x-5.1, the -dev version used a class naming convention that ended up not matching the Drupal 8 class naming convention that was developed later.
Since many, many developers used the -dev version on production sites, the 7.x-5.2 release of Zen uses the same class naming convention that existed in the -dev version. This makes updating from the -dev version to 7.x-5.2 easy.
The 7.x-5.3 version of Zen, however, uses the class naming convention that matches the one developed for Drupal 8.