To a HDD manufacturer, 1 kilobyte = 1000 bytes. So 200 GB = 200,000,000,000 bytes.
In Windows (and most OSs) 1 kilobyte = 1024 bytes. So the above 200,000,000,000 bytes will show up as 186 raw gigabytes or 190,734 megabytes.
Technically, the HDD manufacturers are right.