There is a confirmed bug in MacOS Unarchiver (yet another issue with Macs, so much for them being easy to use!).
You will normally only see this issue when you are using Finder or Unarchiver to try and open the ZIP. The other rare possibility is that the downloaded ZIP file is incomplete, please try the solutions below, before downloading the ZIP file again.
You can successfully unzip via another file manager, such as ForkLift, Transmit, Commander One, or Path Finder instead. These other file mangers do not suffer from the same bug as the inbuilt Finder/Unarchiver app. Otherwise just use the command line unzip program (via a terminal), see below:
Open Terminal from Applications/Utilities (or use Spotlight to find it).
Type unzip followed by a space; drag the file in question onto the Terminal window… the name & path should be completed for you. Press enter/return.
If there are no errors, the unzipped content should be in your Home folder (Users/Your name).
There are a number of third party ZIP apps, such as iPackr, Extractor, RAR Extractor Lite, and Keka (some of these are available on the App Store for FREE) that handle ZIP files correctly.
Do NOT try and mount the IMG file, as no matter what the Mac states it is NOT a Mountable file system or Disk Image, it is the map file ready for use on a Garmin device.