Exhaust Fan CFM Help

Okay, so im building my new grow box and I buy an exhaust fan that goes in a bathroom ceiling and it says it moves 100 CFM (Cubic Feet per Minute) My old grow box had a 120mm computer fan in it that says it moves 85 CFM and I was wondering how is that possible? Or is there some kind of scam behind the CFM's....There is no way the old computer fans did close to what this fan does.

Thanks.

"Fart Fans" are typically larger fans running at a lower rpm than pooter case fans. Properly installed, you shouldn't hear a good fart fan. computer fans are little hot rods. Since the space is so restricted in cases, the fans are designed to run higher rpm, and are growing thicker, allowing for faster moving, "bigger" fan blades. If you cant make the diameter bigger, u can make the case deeper, thus allowing a bit more surface area for the fan blades.

I have never expected the cfm rating of case fans to be legit, and i imagine that since cfm is a selling point for these fans, they may be a bit exaggerated.

I always look for the largest fan that will fit my design so i wont have to run it at insane speeds. cuts down on noise and vibration, and leaves me room for error.