Trademark
"Whisper" is a registered trademark of SSK Industries. In order to sidestep this branding (and/or any licensing fees required to use the "Whisper" name legally), other manufacturers tend to use different names for cartridges in the Whisper family. For example, the .300 Whisper (the most popular cartridge of the family) is often called ".300 Fireball" or ".300-221".
Read more about this topic: Whisper (cartridge Family)