Why is my variable area flow meter not matching up with the Alicat?

Why is my variable area flow meter not matching up with the Alicat?

A Rotameter is a variable area flowmeter (VAF). It provides a flow measurement by using the pressure of a flow stream to float a ball within a conically shaped tube and provides the mass flow measurement of a gas or liquid, when operating in its standard fluid, temperature and pressure environment.

CLICK HERE FOR MORE INFO