It may be the program you are using to unzip the archive. Instead of just clicking it, try right clicking it (or holding down the control key while clicking) > Open with... > Archive Utility. The app icon is pretty much a green square. Starting in macOS 10.9, Apple released their own version of the common .zip file, but added a few layers of security. Trying to open it with anything other than Apple's software will result in an error. They did that to make sure the archive is intact and hasn't been altered. Hope that helps!